Two words: Writer's strike.
Seriously, that's why they were rummaging through old Phase II scripts. Because a prolonged writer's strike was on the horizon.
After only two years? They had just had a strike before the start of season two. I thought those contracts tended to be for a bit longer than a couple of years?
It wasn't about individual shows and contracts. The entire writer's union went on strike through out all of Hollywood, affecting every movie and TV show.
In other words, it wasn't the TNG writers going on strike against TNG after only two years. It was the entire Screen Writers Guild going on strike against all the major studios, including Paramount.
STAR TREK was just one of dozens of shows affected.