I think two things were at play: One, the network was owned partially by Paramount. Two, did you see what else UPN actually had on their schedule during those seven years? As an aside, for me Star Trek will always be Kirk and Spock first and foremost. They were the characters I spent my formative years with. I also tend to think they are the most popular Star Trek characters with general audiences which is why Paramount brought them back when deciding to do Star Trek again.