I would hope that if they were intending to schedule the maintenance at a different time they would give more notice. But maybe I am giving Disney too much credit.
I read that the maintenance they did last year was more extensive - they replaced the cables themselves. So maybe this year they don't need to do as much and the closures won't be as impactful (ie. just closing one line at a time, or just for a few days)?
Or the cynic in me thinks that they know they have trouble filling the skyliner resorts when the skyliner is closed, so are waiting until people have locked in their plans.
Unfortunately I think the simplest explanation is that Disney hasn't finalized their dates for some reason. I believe it's the skyliner manufacturer who does the maintenance, not Disney themselves, so you would think they would have everything lined up ages ago.