It's hard to know... I mean "Young Adult" wasn't really a thing back in the early 90s so I suspect the concern was to differentiate themselves from "Children's" which was a reasonable marketing choice given what was likely to happen to the fan base once the show had been off the air a few years. I wonder if they would have opted for YA if that had been more of a distinct entity at the time - though to be honest I've found the more recent novels which are clearly aimed at that market rather insubstantial.