I personally have no idea why combine vs rebels would be a preferred setting compared to a military one if we're just gonna do faction vs faction combat
same reason gta rp never stuck out to me
project zomboid got zombies and an alternate setting
half life 2 got a combine empire, aliens and an alternate setting
gta rp is literally just real life rp go outside idk
war world 3 roleplay has plenty of makebelieve also, but its too grounded to stick out to me personally
i think roleplay thrives when most parts about it are make-believe, allows you to stretch your own narrative, whereas something grounded has to have mostly grounded outcomes
plus i think the approach has always been different, aside from stasiland, peak ww3 was always about waiting for when you'd finally be let out to s2k, while there was plenty of roleplay to be had; and stasiland was fantastic; half life 2 roleplay has never been about the s2k or even really about battles; but the roleplay surrounding those battles and big events
to contrast this train of thought, stasiland might have died for the same reasons(as in, the way you're saying "why do hl2 outland instead of ww3rp"), because it was essentially half life 2 roleplay, but the stasi and the russians were the big bad union, only diff being that half life is a far removed setting where you could draw your own conclusions, and stasiland characters and outcomes was based mostly on (alternate)history knowledge
not to say ww3rp is a bad setting, its just wildly different