Because people still buy Brock Lesnar as a monster who got beaten by a legend (Triple H) and the biggest star of the decade (John Cena) and The Undertaker is basically the Living GodKing of the WWE, who's had 4 of the best matches in WWE history.
Which why the "oh no, they have to stop using part timers argument and start pushing Bryan and Ziggler and my other wrestling crush of the moment" isn't actually true. The Rock still has a few matches in him and he just drew a million buys against Cena. Lesnar's around for another year. Batista has signed for two years.
In that time, they can push the guys they want to push (Roman Reigns, Big E. Langston, Mojo Rowley, Alexander Rusev, etc.) to the top and keep Bryan, Ziggler, and such as upper midcard or midcard guys who occasionally get a main event push in the off months and work a good match.
In addition, the Network has made each individual PPV worth less. As a result, what the fans "want" means even less, since if you're hooked in by one PPV, you're now on the hook for six months, regardless if you hate the direction.
If anything, the Network means the WWE will be run more and more for Vince's and Triple H's benefit, not what the "fans" want, even though, to be blunt, if the fans want Bryan to be a top star, they should start showing up to house shows he's headlining, so he can start outdrawing the Cena house shows.