This could well be a totally porridge-brained suggestion, but... maybe the warmth of the sets were a factor? I mean, DS9
had a fairly moody look, and much of VOY
was cold and metallic, neither being all that inherently conducive to kiddiing around and having good times, whereas the TNG
sets were bright, cheerful and colorful by comparison. Of course, that could never be the sole factor, but I doubt it would have hurt.
Also, it probably didn't hurt that, after a rocky start, TNG
was a huge hit, whereas the subsequent shows had to live in its shadow.