I come here often looking for solutions, find them, and when I want to upvote the helpful answer I can't because I need 15 rep.
This is mainly because many people come here and think it is a regular "forum". They don't read the help documentation (the help-center, tour, etc) and start using the site in ways it was not designed to be used and that are against the rules. Just like regular forums have new user restrictions, we have some too.
The main reason behind restricting votes comes from voting fraud. You might be surprised how many people will create tons of extra accounts to vote for themselves, just for silly internet points. Because of this the 15 reputation limit has been put in place, to help slow that down.
Did you now that you can get 2 rep by submitting edits on other people's questions/answers? In 7 edits you will have enough reputation to upvote :)