The first reason is Nintendo. They don't produce enough. Maybe of some license Problems? In this case, i am sure, Nintendo do this to have a high value on their products ( in this case the mini NES/SNES ). After 1 - 2 years all the Nintendo Games like Mario or Zelda still have a high value or at least the original store price.

" I blame the stores that allow individuals to buy all or most of the product available instead of using a one per person policy."
Yes, i think this is the second main reason, BUT in my opinion it is ok to buy at least 2. If you can buy 2 why not ? It isn't right to say " no sir, you can only buy 1 piece because other want also one ".
I have bought 5 of the SNES mini, why? Because some friends of mine want them too, but they dont look so often in the store :D So i bought 5 ( not in the same store )for me and my friends , and i am glad i had the chance to do so :)

The third reason ( again, in MY opinion ) is simple and sounds a bit hard: because people are stupid.
People are willing to pay the double or more of the original price, so its clear some other people will use this to get a higher price. Yes the mini NES/SNES is great, BUT its just a console, and if you pay 150 - 200 euros/ dollar, you have to be an idiot. I don't want to offend anybody, realy. I am a collector and i have seen this stupid behavior since 2010. In the last 2 years i sold a lot of my Games, i collected everything, but it makes no fun anymore. I am not willing to pay for NES/SNES games 100 - 500 euros/dollars (Earthbound, Vampires Kiss etc.).

So, if the people aren't paying 150 euros/dollar and more, they don't sell it (at least for such a high price). But as i said, people are stupid and willing to pay it.

So, MY 3 reasons for this:
Nintendo
Stores
People