I thought someone would have answered this by now.
Folks will argue but bitcoin is still a relatively new currency. A couple of regulations here, some stolen money there, and a lot of people may stop using it.
NVidia and AMD's target market (gamers and graphics designers) have a fairly predictable demand. This bitcoin thing is a new category and no one knows where it will be a few years now. Chip manufacturing (unlike say software development) is a heavy investment industry. Investment in factories, people and minerals.
It takes a while to ramp up production in order to satisfy the demand of this new category of coin miners. What if, by the time, the ramp up is completed, the miners have left?
This uncertainty is probably why they are taking it slow. let the market adjusts itself while they ramp up slowly.