Here is another POSSIBLE reason. Airlines schedule their aircraft with great care. The low-cost carriers even more. A plane diverts and unloads passengers so they can be bussed to the actual destination. Aircraft departs empty and when it lands at the next airport it is pretty well on schedule.
The alternative is land at the diversion and wait until it's possible to continue to destination. The longer it waits the later all the flights this plane is due to operate become. If the delay is long the plane may not be able to complete the program for the day because of crew limitations or airport curfews.
No, airlines don't keep spare aircraft for situations like this.