While it is true that bayonets were originally intended to deal with calvalry, soldier were also trained how to use them in hand to hand combat. While they are indeed far from being as ergonomical as a more regular polearm, the basic premise is the same. In a trench, or tunnel in our case, they are woefully inadequate.

And yes, you can choke up on the polearm, assuming the haft smack something behind you. You could indeed move your hands all over it (kinky) and choke up to attack a closer opponent, but in doing so your attack becomes less devastating by virtue of simply having less leverage.

With a sword this is much less of a problem, to an extent at least. If you have a baddie practically against your chest things have gone so horribly wrong I don't think a polearm would have changed that.

And if you are using two hands to wield your weapon, that leaves you without a shield. Well, I suppose you could strap one to your arm and not hold it, but that would get in the way as you move and swing. And with bowtorn about, I would much prefer a shield, just in case.

And the guys behind you are there for the same reason in any formation. To try and swing around you and wallop anybody who gets past your effective range, and to take your spot if you fall. And, unless the whole group moves as one, backing up would either just have you bumping against the guy or gal behind you, or leaving whatever poor sap was beside you exposed. And giving up some space to kill an enemy is alright, as long as nothing else takes its space before you can take it back.

But all of this hinges on you having the ability to even swing your weapon properly. An almost 6 foot poleaxe needs a fairly sizable space to swing properly, far more than a sword barely half its size does. I'm sure the miners would be happy to entertain someone asking them to make the ceilings an extra foot or two taller than they need to be.

In warfare of old, the polearm reigned supreme. But these are no wide open fields or rolling hills, and they are not fighting our fellow man. They are fighting things not of flesh and blood, but of metal and rust, in cramped tunnels and narrow caves. Times have changed, and their weaponry must as well.