It's a complex game, people for the most part do not care about 20x20 or 10x10 in teamgames. What they are worried about are:
1) do I have a simple role?
2) do my teammates have a simple role?
3) Is having a bad player generally recoverable?
4) Does the map induce getting to the fun part of the game? (subjective, majority would say that is late t3-t4 stage).
Games similar to FAF (Zero-k) have the exact same problem in their custom lobbies, except to an even more extreme. They can do like 16v16(?) games and so most of their lobbies end up becoming that because, well, it makes the game simple, easy, and let's you quickly get to late game stuff because units are essentially pointless.
There's also a hosting economics question that comes into it with players rather joining a 7/8 game or an 8/8 game and begging the host to increase game size rather than creating a new 4v4 lobby. It's the easiest way for me in this scenario to get into a game as a player. This mainly relates to wonder hosts going from 4v4 -> 5v5 -> 6v6 -> 8v8. Certain maps have a "predefined" player quantity that doesn't adjust to this fluidity and that's mostly determined by community opinion I guess.
On the question of "Why do people not play bigger maps in 4v4" the answer is ultimately found in weighing the fun you would get from such a game with the possibility of it filling. And even if it does fill, most of those that would fill the game would just be garbage anyway. This leads you to the conclusion that you might as well as play a 10x10 where Sui dumping all mass into mantis and walking into enemy acu will at least have the off chance of being productive.
Or just wait for the weekend when you can ping enough people to get a good high rated 3v3 going on a nontraditional map.