Why not only make tent poles? Well, they try, which is how we get endless superhero movies.
Why not just make up a new tent pole? Because no one can figure out in advance what they are. No one in Hollywood seems to be able to, ever. Look how many "sleeper" hits were considered unimportant, and how many big budget, highly promoted films fail.
Hollywood doesn't know what's a hit until after it's made, and neither does the viewing public. Great art with lasting value can't be crowd-sourced.
Voting on which shows to greenlight will probably select for the flashiest pitches/trailers, without any effect on the resultant quality.
Also, generally speaking people don't know what they(we) want. It's hard to like something you've never seen. New shows are full of new stuff so how can the public make a good evaluation after a 1/2 hour or even a full hour.
I don’t know why they stopped doing it.
You will also end up with less ground breaking shows and more mundane stuff which is already the problem at Netflix.
“If I had asked people what they wanted, they would have said faster horses.”