Well, the reason why they can't just pay for Clarste's work is kind of three fold? Putting aside the issue of 'why don't they just release it at a super-speed cycle," which is its own can of worms (that boils down to 'wow they would burn SO MUCH money never in a thousand years')
First off all, there's the issue that they'd have to spend time and effort on trying to contact Clarste and sort out a deal when...they already have a translation team that they pay to handle this stuff. Second issue is, well. Said team probably isn't keen on yen press turning to fan translators, it might start digging into their own work safety. And thirdly, and this is probably both the most important thing...
With all due respect to clarste, and I'm eternally grateful he's translated the work so far - his translation job has issues. Things like Mamizou...not really having any sort of accent for awhile when, in the original text, she's almost like a grandma. To what extent the translation's flawed, I'm not really qualified to say, and again I have to clarify this statement by saying I love the fact he translated this stuff for us and I'm not trying to bash the man's work here, but for something as short as a print work I don't think there's a snowball's chance in hell of a company that already has a paid TL team on staff trying to go through the trouble of getting a fan translation even if it was utterly perfect, so a fan translation (which understandably so) that has flaws has...well, a negative chance.