A single up vote on an answer is worth 10 reputation to the receiver, and on a question it is worth 5 reputation.

Giving an up vote is completely free to you, and is more of a bonus than a single bounty of 5 reputation.

As bounties can only be given to answers that makes your proposed 5 rep bounty worth less than a single upvote.

There is no reason to give small bounties when the reputation system does it naturally, thus a minimum bounty of 50 signals a "this is actually good" intention beyond what a normal upvote is doing. 

Being able to give 5 rep would be a slap in the face. "I didn't want to upvote your answer, which costs me nothing, so here's 5 rep to ride you over instead."