I think it's partly because I know that Miles is a perfect example of a good Slytherin, that part of me feels duty-bound to write a "not all Slytherins are evil" epic because that was one thing I was really disappointed about in DH, but another part of me groans at the huge amount of work that would require, and I'd rather do something else. It's sort of like I have an opportunity here to redress an injustice, so there's somewhat of a "should" about it, which makes me feel guilty for not doing it. Which is bad. So I'd rather avoid the guilt by avoiding continuing this crossover altogether. Irrational, I know.