The biggest reason is that I get more offers for work than I can complete within a month. Chances are, there's nothing wrong with your proposal, I simply don't have the time to accept everyone's.
My selection process usually involves reviewing proposals to check if they're something I'm actually willing to do and those are sorted into a 'maybe' folder. There's usually a couple that I think are really cool and they go straight to the 'accepted' folder along with any in progress sequence pieces. The rest get written on a sheet with the fetish type and cost and I pick through them to get a decent spread until I hit what I want to earn for the month. There are some people I prefer to work with, but in general I try to vary commissioners and accept at least one new person each month.
Usually the things that get a proposal rejected outright are content I won't do (e.g. nudity, futa), written in a very vague manner (e.g. a character reference and "you know what to do"), asking me what I want to do (it's a nice thought but I'd draw it for myself anyway), or written in a way that suggests the commissioner won't be a good client (e.g. demanding finished work before paying). These are all pretty rare, and I'm not going to dismiss a proposal for missing the e-mail, asking me if I think option A or option B for their proposal would work better, written by someone ESL, or for saying they can't pay until the following week.
Another thing is people sending notes asking if I'm open or closed for commissions when the information is literally on my front page. That kind of implies they're not going to read terms.
Commission info is here, by the way: Commissions Closed.I have to ration my time somehow, and I believe this selection process works better than raising my rates - for now.
I'll answer general questions about selection but I don't want to respond to specific "why didn't you accept mine" inquiries.