I don't understand why you can't place a bounty until you get 75 rep but the minimum bounty is 50. Wouldn't it make more sense to be able to place a bounty when you have 51 rep since you have enough rep to place a 50 rep bounty and you will still have 1 rep left so you don't go under 1?
So, I don't know the official reason why bounty is at 75 and comments are at 50, but some guesses (I suppose I might as well make an answer out of my comments):
It makes sense to me that you'd want to give new users the experience of leaving comments before you give them the experience of setting bounties (re), especially given that leaving a bounty can cost you the privileges. If you could leave bounties but weren't allowed to comment, you might be encouraged to peg your rep low leaving a bounty to get more attention to a question.
I think it also might be confusing, too: Commenting on stuff is a ubiquitous action anybody who uses the internet is familiar with, but bounties are more of a niche thing. So it makes sense to me that you'd let new users do something they're familiar with before you let them do something that's a specialized feature of SE sites that they may not yet understand.
Also having the privilege in the middle there, unrelated to bounty behavior, is nice because earning privileges feels good and 75 is a good spot to strive for. It adds a little bit of encouragement to the initial rep gain for a new user on a site. If bounties and comments were both at 50/51 and the next was at 100, there'd be a little gap there where you're faced with the seemingly daunting task of doubling your rep to get a reward.
I don't really think it's too much of an issue as it doesn't take a prohibitive amount of effort to get that extra 25. It'd be different if bounties were at, say, 500 or something.
At 50 reputation one gets the Comment Anywhere privilege, and at 100 there are two: Create Chat Room, and Edit Community Wiki.
So to add extra "level", they decided to put it in between.
Another motive is that one can't have reputation of 0. If we could start bounties with 50 reputation, it means we'll drop to 0, which is impossible. And awarding the privilege at 51 reputation would mean dropping to 1 reputation, which is too low, losing all privileges in one go.