Making 32 vs 32 TDM in Portal would be no challenge. I can do it. But would you play it? Not unless you could matchmake, right? You wouldn't take your time shooting bots while waiting for it to fill up, or tell other people about it, or tell people that portal is a good thing, or convince people that portal exists, has a server browser, and you can use it to find that 32 vs 32 TDM experience, right? If the players didn't just funnel in naturally, it would not be fun and would be a waste of time creating what you want, yes?
I have a 16 vs 16 Hardcore TDM experience that I made. Hardcore TDM not being an option normally, is its draw. Custom spawns, scoreboard, and win tracking. It has filled up a couple times, and that's it. No one plays it. I would have to be convinced that people want to play 32 vs 32 TDM enough to take the time to play it.