Frankly, it was all contractual, & union related. They had regular recurring cast that were contracted to parts based on lines, such that Gates was only there in the same capacity as say Lt. Monroe from Disaster
who had a few lines & then died.
Except, Gates doesn't die. Her & a few others stick around, because presumably there was room on the payroll for some regular one or two line actors, which apparently is still a good paying gig in tv work
I'm actually amazed that even one of them broke out into something more, Alyssa Ogawa. Perhaps they fought a bit to have one expanded some, & if that's the case, I'm glad they chose Ogawa, because there's lots of speaking characters on the bridge, but sick bay had almost none