Um, I'm not sure where you were located back then, but for me in Southern California at the time KCOP-TV was the home of Star Trek Next Gen and Deep Space 9. When KCOP turned into UPN, it was no surprise it continued to be the home of future Star Trek shows because the network originated in Southern California.
Well, I remember at the time UPN being an odd fit because of its target demographic. Maybe that was everywhere else but SoCal.
Here in DC, Star Trek stayed on the same channel too - "technically." it was WDCA-TV 20, and it turned into UPN just like how you're describing. And yup, TNG and DS9 were on the same channel. It was the other shows that changed, as well as the marketing of them. Pre-UPN, I watched that network all the time when I got home from school; they played repeats of sitcoms like Family Matters, Full House, etc, and then at 7 PM they showed TNG repeats. Had it on every day.
Then it switched to UPN, and all the shows I liked watching started to gradually disappear. There were a couple shows I liked on UPN besides Trek, to be sure - The Sentinel, for one. But by the end, it was all Girlfriends and WWE Smackdown (I'm not quite sure who would watch both of those, but I guess I'm not them). I remember seeing the network commercials in the last two years of Enterprise and thinking "WTF is this crap?" But maybe that's just me. Perhaps if you were within their target demographic, you wouldn't have noticed. That's pretty common - when you're being focused on, you don't realize that it's not for everyone. I'm not trying to insult you by any means, it's a fallacy we all share.
It's funny, since UPN disappeared, FOX bought the local station, and now it's back to repeats of shows I like (LOTS of The Simpsons, for one, and some Big Bang Theory) and the occasional Redskins game for the local market when the game's on ESPN or NFL Network. So I tune in again!
