i already said i am thankful for those things but you need to realize TNG had large amount of viewers because there was nothing competing with it on TV at that time.
Voyager and Ds9 were going against each other as well many other Scifi series in 1995 and later.
Damn... you're right. I forgot that from 1987-1994 there was only one TV station here in the States and folks were forced to watch TNG or nothing.
Many of the shows that you complain had to compete against each other got on the air because TNG proved that adult sci-fi was a marketable commodity. And as far as I know, DS9 and Voyager never had to compete head-to-head for viewers. One aired mostly on the weekends in syndication while the other was on a network (UPN), Wednesday nights at 8pm.
DS9 and Voyager simply didn't engage mainstream audiences the way TNG did. TNG didn't suffer a ratings decline while it and DS9 were on the air together.