I think a lot of people are hesitant about making stylistic suggestions because they feel that style is "personal" and therefore commenting on/criticising someone else's writing style comes off as a personal attack. Which I think is insane, but so many fanfic writers are so "sensitive", it's not surprising that a lot of people can't be bothered to take on the drama. That observation is based on my experience in HP; other fandoms may be different.

Part of the problem, of course, is that to get good quality crit, your beta has to be at least as good a writer as you. Or they have to possess a better understanding of how things should read, and the mechanics of good quality prose. And if you're a proficient writer to begin with, that makes the task of finding a truly helpful beta that much more difficult.