Currently, Fox has deals with both the NFL (for Thursday Night Football) and the WWE (for Smackdown), and since the network doesn’t utilize the same three-hour primetime blocks as its biggest competitors – outside of football – programming execs had to shake up the prior status quo.
The most basic version of the answer is that Fox booked up too many weekly programming hours for its fall months, and there just wasn’t enough room for Last Man Standing to properly get slotted in, forcing the extremely popular comedy back to the midseason schedule.