- .. what explains the lack of pluralism in the current academic landscape?
A) This is mostly due to empirical revolution in economics.
In the past economics used to have a lot of schools of thought (e.g. Keynesian, Marxian, Classical, Austrian etc), which coexisted along each other. This was in non-trivial way due to the fact that different schools made different theoretical assumptions in their model and in the past there was no way how to test those assumptions (for example Keynesian view on shape and behavior of aggregate demand and supply vs classical or Austrian one).
However, as you can see below in the picture from Angrist et al. (2017), during the last century economics underwent empirical revolution. In most fields most papers nowadays are empirical.
Empirics in turns weeds out theories from science that cannot be empirically verified. For example, in physics there was a decade when there was Newtonian "camp" and Relativistic "camp" of physics coexisting, but soon whole physics shifted to relativity once Newtonian physics was shown not able to explain empirical observations (or more recently in physics there used to be debate between dark matter and modified Newtonian dynamics but now dark mater is physics orthodoxy thanks to empirical observations).
Simply put, once you start empirically testing things scope for diversity naturally declines. For example, if labor theory of value continuously fails multiple empirical tests and does not offer useful predictions and explanations why would you take it seriously?
Of course, no theory is perfect, there might be instances when theory fails some empirical tests but it is still used because there are no better theories to replace it, but generally speaking if some theory fails test after test, then it is just matter of time before it gets marginalized and weeded out.
B) I would also challenge your assertion economics lacks pluralism
I do not think economics has a lack of pluralism compared to other scientific fields. In fact economics is probably more pluralistic then let's say medicine, physics or biology.
For example, take Marxian economics, which empirically has empirical success on par with homeopathy. Marxian economics is still being taught and discussed at some universities, for example at the University of London. It is just no longer prominent in the field. However, in medicine you would be much more hard pressed to find actual credited universities teaching courses in homeopathy.
Yet you will find even some good ranking schools, like Institute for Social Studies (part of Erasmus University of Rotterdam), that teach Marxian oriented programmes.
I would like to see any serious study that would actually show economics is not pluralistic compared to other science fields. I would even dare to say that economics is one of the most pluralistic fields outside of perhaps sociology or philosophy.
However, in any field of science, scientists will eventually arrive at consensus when it comes to positive results. E.g. why would anyone still teach Ptolemean model of solar system outside history of physics, or in economics in Physiocratic theory that all value is derived from land and land improvements only outside history of economics.
(2) Do you know of any post-keynesian and/or marxist courses currently taught in Canada or USA (at a graduate level)?
Yes here is a list of universities that have heterodox PhD economics programmes. Some of them are European but there are also American universities there. For example,
American University, US
The diverse theoretical approaches are combined with solid training in empirical methods which prepare graduates for teaching in colleges and universities, research positions in government departments or consulting firms, and policy making. Our Washington D.C. location gives students excellent access to government agencies such as the Bureau of Labor Statistics and think tanks such as the Economic Policy Institute and the Institute of International Economics. These agencies give students special opportunities for internships and part-time employment as well as the chance to hear and speak with economists dealing with today s national and international economic issues.> The Department of Economics offers a Ph.D. degree with programs of
study in either political economy or traditional economic theory. The
neoclassical and Keynesian traditions form the core of economic theory
taught by the Department of Economics. Our uniquely pluralistic
approach to economics encompasses a range of other perspectives,
including Post Keynesian, Institutionalist, and post-Marxian economic
theories. Our program emphasizes international and economic policy
perspectives. Specialized course offerings include the economics of
gender, the economics of transition economies, economic methodology,
monetary economics, public finance, economic development, labor
economics, industrial organization, international trade, international
finance, econometrics, economic history, and mathematical economics.
Colorado State University, US ↑
The graduate program of the Department of Economics integrates rigorous training in quantitative methods with a broad, historically-grounded and critical approach to research and teaching that encompasses a plurality of perspectives and streams of economic thought. M.A. students are required to take core courses in each of the fields of macroeconomics, microeconomics, econometrics and political economy. PhD students take an additional advanced course in each of these fields. Beyond the core, students have a great deal of flexibility in selecting their fields of emphasis and research. The program has two main foci – political economy and regional economics. The heterodox political economy component of the program has traditionally been active in the fields of radical economics and institutional economics. In recent years, this dynamic and evolving program has been complemented by faculty working in the fields of feminist, structuralist, post-Keynesian, and Marxist economics, with an overall focus on international economics and economic development. This foundation prepares students for research and teaching positions in colleges and universities, research positions in government and the private sector, as well as for policy-related work with labor, environmental and international policy organizations.
PS: Steve Keen is discredited scholar (see this old SE answer). Teaching Steve Keen in graduate school would be equivalent of teaching the Rogoff & Reinhart theory that once debt-to-GDP goes above magical threshold of 90% country will have debt problems in graduate macro program. Although I am not even being just to Rogoff & Reinhart because they had only one seriously controversial research project. No matter how much someone hates mainstream economics, if S. Keen is the alternative then good grief!