Why is it, in the all the battles we've seen involving multiple factions of Black Mountain, we've only ever seen one Beholder take part - that being Bob? Do the Beholders wish to stay neutral no matter what? Do they simply not give a shit who runs things? Do they have their own plans in the works that fighting would only interfere with?
To go a little meta, think about what happens when Bob shows up: The side he's not on loses in about 30 seconds. If you had more Beholders coming out for battles, they'd end things far too quickly.
Beholders in Dungeons & Dragons, despite their nominally lawful alignment, are aberrations. Blue and Orange Morality doesn't even begin to cover it, they don't even have the same basic needs as other forms of life (e.g. air, food, and water). From a more mechanical perspective, they're essentially walking weapons of mass destruction, with even relatively young/weak ones easily capable of tossing out as much firepower as as six to eight almost max-level wizards going all-out at the same time, with the additional ability to automatically negate magical defenses and block spells coming back at it.
I suspect it's mostly (2), mixed with in with the fact it seems that orcs, goblinoids, and drow are by 'far' the most populous races, and are thus much more visible, point blank.
Probably a mix of 3 and the first answer(it'd be too easy), since there seems to be a good load of them in Black Mountain. Also, they do have a King Beholder, as seen... in some strip where they were deciding who would lead the mountain after the Bugbear King was defeated.
They also don't seem as ambitious or warlike as a culture as the other evil races.
What's with the drow crumbling like vampires if the sun touches them? Blinded I could understand, but dusted?
Quite possibly this is a reference to what USED to happen to drow weapons and armor way back in the TSR D&D days, supposedly anything made by a drow crumbles into dust in sunlight, plus Rule of Funny .