Because of population growth, savings, and technological innovation the stock of capital and augmented labor are growing over time. These forces raise the marginal product of land. Unlike capital, the supply of land is fixed. This means that the supply of land can't increase in response to the rising marginal product. Investors have to be indifferent between investing a dollar in holding land and a dollar of productive capital. Capital is paid its marginal product ($r$). For investors to be indifferent, each dollar buys $1/P$ worth of capital, and that fraction of a unit of land returns $r$. One unit of real estate actually returns $rP$. So, over time, the forces that raise the marginal product of labor and capital, population growth, capital deepening, and technological innovation, raise the marginal product of land and therefore push up the price because the quantity cannot adjust.
This assumes we can abstract from issues of taxation and depreciation. We can handle those details but they don't change the basic story. What is more important issue is that land is in fixed supply and capital is not.
There are some innovations that increase the literal or effective quantity of land: e.g. landfill, multi-story construction, enhancements in in agricultural productivity. More accurately, in practice, the quantity of land can adjust can adjust in a limited way, more slowly, and at greater cost than physical capital. This preserves insight above: quantities do not adjust to equate the marginal product of physical capital and land, and so the price must do so instead.
