Short answer? Competition.
If someone bought a NS say, this time next year. You can pretty much list out the top 7 games the person can or will buy right now. Now since there are very few must have games to chose from or more specifically fewer publishers offering must have games on the platform, nintendo pretty much has a monopoly on their library. The know that anyone buying their platform is going to buy (the usual suspects) mario, mario kart, smash, zelda.....etc. The same group of best selling games on every nintendo platform. So the only reason to drop their prices is if their sequels are released on the same platform.
Its also a way of forcing the hand of their buyers. Take me for instance, I have not bought either detroit or spiderman yet, because I have a backlog I am still going through and I know that by the time I am ready to play them sometime in march next year they will cost less. Hence it makes zero sense me buying it now and having it just sit in my hard drive till next year. Nintendo buyers don't have that luxury. You might as well just buy the game now cause its price is never coming down. Meanwhile when I am ready I will probably get both games for $60.