Just to be clear: so far we don't know what the two novels are about and when they're set. They could just as well be set in another time-frame, but ...
If it has to be TOS stuff why can't it be something interesting? A Arex novel? A M'Ress novel? Something continuing Christopher's post TMP storyline? Captain Spock before TWOK? Something with the crew of the Enterprise-A?
Not the 500th adventure of the five year mission.
The easy answer? 5YM novels sell. I would like if they would throw in more out there TOS concepts from time to time, but quite frankly I wouldn't be too surprised if the TOS 5YM sales supplement some of the 24th ongoing stuff.