It's the exact same problem on UHC, people don't understand that if they just stay, the game will start, they just leave and try to join another game.
Sorry but no one really plays that game anymore except for few people who're willing to just play for the leaderboards. If you play that game it's probably because of the fact that other games are quite boring.
Just had to say that. On topic though:
This is the problem with every game. Players. UHC started with 30/40 players max(Not sure). They could've easily gone the Hypixel way and made it a 100. Why? Player population. Battlezone is a fun game, but with 100 players playing. After a few weeks, the lobby would have a max fill of 60-80. Slowly, this number will keep degrading.
To fight this:
Lowering the player count needed to start a game would be a good solution as you mentioned. The hard part is finding that sweet spot where the game doesn't feel really empty, and to have a good experience.
Another solution to attract players is to have a multiplier event where exp/points (either one) is doubled. It's sure to attract a lot of players.