Easy mode for people who don't care about fun and just want to boost their stats with decks that are so braindead that literal children can get to top-tier rankings with little to no experience.
The portion of players who can efficiently make money by playing Magic they don't enjoy is much smaller than the portion of players who play blue decks. Therefore, a lot of them (and I know that this at least includes myself) care about fun, or else we wouldn't be playing Magic.
I'm guessing it's because Wizards wanted it to exist for constructed, and it was not useful in limited. Commons and uncommons are mainly designed for limited, as they're the cards you see the most in it.
There is no such thing as an inherently "scummy" card. If a card is legal in the format you're playing and you and your opponents have not agreed not to play it, it's fine to run.
Where did you get "70%" from? Which formats and events are you claiming this is the case in?
The portion of players who can efficiently make money by playing Magic they don't enjoy is much smaller than the portion of players who play blue decks. Therefore, a lot of them (and I know that this at least includes myself) care about fun, or else we wouldn't be playing Magic.