They want to make sideboarding matter. Bladetusk Boar is "random" because it just says "if youre opponent isn't playing red, I'm probably unblockable. If they are, I'm not." They want cards that more consistently function the same in match to match. And for color-hosers, that means making them sideboard cards you bring in. Cards that turn off or on arbitrarily based on matchups with no intent required don't add skill to the game, they add luck/variance, and they add it in a place you probably don't want it.
Jund has flying in Black, Falter in Red, and Trample in Green, for evasion, btw.