The problem is the colour imbalance. Like you said, red is the worst colour because it has neither consistency or power level. You can run Gamble but a percentage of the time it's going to blank. When any black deck can run a handful of tutors for a marginal cost (less threats), it feels a bit unfair. Green also has this issue with Tooth and Nail which is a tutor and combo in one. Banning Demonic Tutor is probably too reactionary, but something like Tooth and Nail definitely deserves to be on the watchlist.I'm pretty sure that's an interesting argument the people who maintain the format have had more than a dozen times.
I'm honestly not quite sure what the right approach is. At the very least I could get behind banning the super cheap (in terms of mana cost, and conversely super expensive in terms of real money cost) tutors for the same reason cards like Ancestral Recall and Time Walk are banned. Maybe just blanket-ban every tutor under a given CMC.
It's a somewhat complicated problem, though, because a lot of completely fair decks still want tutors badly, so removing them might winnow down the field of viable decks rather than expanding it.