Affinity becomes too powerful an ability when you get to count most of your lands, as do Arcbound Ravager and Disciple of the Vault, and there are also older effects like Goblin Welder that can make the situation even worse
They have very similar effects, with both forcing the owner of a permanent to shuffle it into their library, and then they reveal the top card and if its a permanent they can put it on the battlefield
They might deal with a few, they might stall a couple for a while but every turn is too much to beat unless you are very ahead to begin with
Sure, you can tutor it up but then it is even slower and more costly to your tempo
Well, I can actually see whyits not legal in Pauper
Players didnt care for their valuable cards becoming worthless overnight, and R&D developed the Reserved List, a list of cards that will never be reprinted or have a functionally identical version printed, in response