There's not enough trade_plaza maps, guys.
I've been around way too many tf2 servers, as I'm constantly changing communities when the current ones have gone to shit.
Popular maps that I've seen (that won't be added I bet) are custom degroot, Slender Fortress (which might not even work, because there are a bunch of empty SF servers out there that NO one goes to since Glubbable's is the best and most frequently updated. But if you take VIP, make it pretty much free, and revise the credit system, as well as add some really cool things that MAKE people want to stay, then you have a chance.), Death Run, and Fun Servers (Silly commands, fun maps like the zelda one or old imported maps from the Nitendo 64 (Banjo Kazooie, sanic, etc)). Fun servers also usually consist of custom models, which is really frikkin' awesome. You know, awesome - that thing that doesn't exist here.
One of your combat surf maps could be replaced with skillsurf (with a bunch of beautiful maps, and more importantly, Bhop), and the other one could use a severe map update. Surf_Rainbow sucks ass, and is a bitch to beat. No wonder it's empty a lot. Yes, there are other combat surf maps. I have almost every map known to surf in my brick of a PC.
These are servers that - if not filled to the brim with players - are never actually empty either.
The only reason Saxton Hale, Slender Fortress, and Death Run are so "popular" to you guys is because that's probably all you have ever been on, and more hilariously, you only went to the few player-filled ones, and not the rows and rows of empty ones wishing for the players it never had.