Bluntly, it's the stupid maps. They're made for 10x the amount of players that we have now (I'd wager that even with 10x the amount of players, we'd still have room for more). Fighter Ace suffered the same fate and it went down the exact same path that Aces High is starting down now. It started with decreasing numbers and making it difficult to find fights.
This is why people love War Thunder and WoWP - it's not because of superior gameplay, it's because it's solid, repeatable gameplay. You can find a fight anytime you want.
Decrease the map sizes, compensate for fewer strats with higher defenses and game mechanics. It's fairly simple.