Because some people think they have enough prediction skill/competitive skill to say "Oh yeah I totally know how this game works" sadly. I mean yeah I'm upset by the ban, I think this hurts a good amount of decks people use that just aren't top tier enough but at least competitive. I know this hurts me because with my main deck, I found decking out was very harmful/likely to happen because sometimes I needed those risks and the only way to at least stand a chance was dropping my whole hand with Juniper. When Trump Card came out I was very happy because it helped me plan ahead in case I desperately needed it.
However I'm hoping, because I DO love this card game, and wish it would become significantly balanced again - that this was thought out with the future sets in mind. Because with two sets being revealed right around the corner there has to at least be a better reason or alternative to ban this in order to make things balanced again. But because we DON'T know what to completely expect, predicting the format ahead of time has always been a pet peeve of mine. The fact that this ban doesn't go into effect immediately could mean there's something, or it could just mean nothing. Either way, I'm not gonna jump on this format prediction bandwagon. The only time to ever try and predict a format should be when it's already happening and you want to prepare a deck for it.
I've seen people who instantly think something is broken/competitive the moment it's revealed and then build the deck, only to find that they had no clue what they were doing because they didn't know enough about the game.