What QL and Miranda said. The other thing I'd like to point out is that many beta-readers are probably more timid to offer when the request is for a mentor instead of a beta.
A true mentoring relationship is not only a bigger time-sink than beta reading, it is also asking a stranger to take on a larger, formative role in your learning process. When someone asks for a mentor, they're asking for heavy advice and practical experience, to be guided through a process; they're asking to receive without having to give.
Specifically, you're also asking for someone to walk you through not just the writing, but the self-publishing process. There are so many places to get self-publishing information and walk-throughs without asking someone to recreate the wheel in an e-mail to you.
When someone asks for a beta, they're asking for reader experience and suggestions. And while not all betas swap (I'm sure many on this forum read more than they ask to be read), it's possible that betas don't want to walk into a situation where a mentor-student responsibility is expected of them. Personally, I wouldn't want anyone to take the comments I've made on their MSS as law. I'm one reader. This happens to be what I thought about this thing. That's all, I'm not right or wrong, and I wouldn't want to be pressured into providing advice knowing that the person taking it is adding weight to it, treating me as a teacher instead of a peer.
A critique partner or beta reader is a peer. Most writers here, I'm guessing, want peers.
That's my .02. I feel like we've seen an upsurge in mentor requests lately and they don't get quite the positive responses as requests for betas. I'm sorry this went a little long or hijacked your thread at all, JB, so let me give you a suggestion: look for betas not a mentor. And glance through the rest of the beta seeking threads and see what kinds of information they are giving to get volunteers. We need to know enough about the story to know if it sounds like our kind of thing.