Apparently the gajillion posts about "Fraction sheds readers" wasn't enough. Sabrewulf had to make a whole thread dedicated to it. Thumbs up for monotony.
They're doing it for the same reason they paired up Bendis/Maleev on Spider-Woman and Moon Knight (and yes I realize both fell down sales-wise), they're hoping to rekindle a writer/artist duo that has put out good comics in the past. Hawkeye is not so much about putting Fraction on the book, it's about putting Fraction/Aja on the book.
Also, the comparison to Ultimate Hawkeye is so insensible that it borders on trollish. That said, I'm sure it made sense in your head. But as someone who fancies himself so well-versed in sales figures....I'd figure you'd be aware that miniseries, aside from big events, almost always have significantly lower sales than comparable ongoings. There's no comparing the two. The fact that it's the Ultimate universe also makes it a different can of worms.