Because it's hard to do
In the multi-wizard fights where we don't see Expelliarmus, the reasoning seems clear. You need to K.O. your opponent swiftly and might not be able to follow-up with another attack because you have to go fight somebody new.
Expelliarmus is both a blockable spell AND one simple enough for a twelve-year-old to learn. So presumably the reason you don't see it being used all the time is that the counters to the move are simple and obvious. It works for Harry in multiple instances because, reading between the lines, the move is so unlikely to work in a life-or-death situation that nobody actually expects someone to try it. Ergo it works for Harry in specific scenarios where his opponent is either overconfident (Voldy in the graveyard), surprised (Malfoy Manor), or under the Imperius Curse (Stan on his broom). In the first case He-Who-Must-Not-Be-Named is clearly not expecting much of a fight, and the fight he DOES expect would be one that he (never hesitant to kill) would make. In the other two instances it's fairly clear that other spells would have been more effective (Lupin straight-up tells Harry so regarding Stan in Book 7).
Even assuming the spell isn't as easily blocked as it appears, it's still clearly not a top-tier combat spell. You yourself said that Expelliarmus would be most effective when combined with another spell. But that means that in a fight you have to hit your opponent twice to get the same result as a stunning spell. Inefficient, and dangerous. To make it work you'd have to have both spells castable as quickly as possible. So, if you fire off Expelliarmus, you don't have time to see if it's worked—you gotta go right into Levicorpus. But if you missed, your opponent can now hit you back. If you take a second to see if they ARE disarmed, they can hide behind cover, grab their fallen wand, or perhaps (as we know you can at least do Lumos when the wand isn't actually in your hand) even cast something at you if their wand isn't too far away. And if you hit them with Expelliarmus you then need to cast ANOTHER SPELL to incapacitate your opponent. If you miss that one, they can get their wand, tackle you, throw a Neezle at you, whatever. At the very least you need to hit your opponent with Expelliarmus and then grab his wand as it goes flying through the air. It's extra time dealing with your opponent and that leaves you open to attack. Meanwhile with something like Stupefy or the Killing Curse the enemy is out of action, first hit, guaranteed.
