Problem: Nobody can agree on which maps are good. It's actually impossible to find a map that everyone likes, just look at the responses in this thread (and elsewhere). Some people hate Badlands, others hate Bagel, others Gullywash and Process, etc. etc. You will never have a map pool that satisfies the community. As a tie-in to that, people are divided on if KOTH is good or if 5CP is good.
Problem: TF2 players are allergic to trying new things, specifically maps. See the participation in various New Map Cup events over RGL's history. Their immediate reaction to a new map is "it sucks because it's new/weird" or "it sucks because I don't know how to play it." Turns out you have to actually play a fucking map before you know how, crazy.
Problem: If you make non-Invite divs run pick/ban, I'd wager teams would just ban all the newer maps and repeat the same three to four degen pug maps all season long. Oh wait, I don't have to wager because ESEA did that already and we saw how that turned out.
Problem: RGL's methods for handling the map pool are absolute shit. Need I remind everyone that they seriously tried to claim that people hated Granary after last season because it was the least popular map from a SEASON 2 POLL. Then they don't run Granary in the most recent New Map Cup. They could actually do proper things to get the playerbase's opinions on maps or facilitate new/old maps being introduced but nah. RGL not using common sense yet again.
So what we have are a myriad of problems and RGL isn't helping anyone with how they're going about it. And we will continue having this discussion until the end of time. Man, remember when in ESEA we just voted out Granary and then voted it back in? Good times.