Why do people play 40K?
Do they play it because of the rules?
No, the rules suck.
Do they play it because it's a 28mm game?
No, there's a bajillion 28mm games out there,
Do they play it because it's a sci-fi mass battle game?
No, there are plenty sci-fi mass battle games.
Do they play it because of the 40K lore?
Yes, Warhammer 40,000 is the only official game based on the 40K lore.
Do they play it because it's the only game at the FLGS?
Yes, but playing a bad game with >$300 buy-in costs because it's the only game available is incomprehensible to me