I have no knowledge of whether this is accurate or not, but when MIA let go its mainline employees, there were posts commenting that this was a contractual thing -- MIA had been designated an express station, and a mainline aircraft could only operate there up to five months of the year; if it operated any more than that, UA would have to put in mainline employees.
If true, I'm not sure why they wouldn't just switch it to a UX route for the rest of the year.