The Crusaders will never get cutie marks because marks are a manifestation of Harmony, and the crusaders are equally in the grip of Chaos.
This mean most ponies find them unnerving.

Sweetie Belle is usually over at Rarity's place because, despite being a nice little filly, they're kind of scared of her. Same with Scootaloo, living with her aunts, away from her parents, and mostly spending time even away from them. ( Agreeing with >>42286958 ) Applebloom lives at home because ponies of Ponyville are tough, living so close to the Everfree, and probably have a touch of Chaos, themselves. (It's part of the reason Discord chose it as his new capital.)