I wonder if permanent cast size had anything to do with it. After all, the Doctor only had one companion at a time - usually - and if you weren't really a fan of that companion (or, as some feminine psyches might interpret a female companion as 'competition' at some subconscious level), the other alternative was to self-insert oneself into the fiction. Trek offered a wider variety of cast options and a lot more in the way of romantic subtext (unless you were reeeeeally reaaaaaally projecting/looking for it, with the possible exception of Romana II) that just wasn't visible in DW; not until 1996.