The answer to this question isn't 100% straightforward. The text doesn't explicitly explain why Maniac turns down the offer from Mars Bar. We just see Maniac struggling with the offer and finally just admitting that he can't do it.
"I can't. "
"Why not:'" said Mars Bar. "My house not good enough? My mother?"
Maniac struggled for words. "I didn't say I didn't want to. It's don't know . . . things happen . . . I can't . . ."
Mars Bar is not okay with the answer. He gets upset with Maniac, and Mars Bar attempts to ask Maniac one more time. Mars Bar even suggests that Maniac just try it out for a bit. Maniac doesn't accept the offer and walks away. The following chapter has Amanda telling Maniac that he is coming home with her to stay, and she doesn't take no for an answer. Maniac isn't jumping for joy at the situation, but he does go with her.
I think Maniac's main issue with going home with Mars Bar (or anybody else) is that he is afraid of the situation. He has been alone for a long time for sure, but I think that is because he's afraid of the family dynamic. He hasn't had a great experience with families. His mom and dad loved him, and that was a good situation, but they were killed; therefore, Maniac had to deal with that tremendous emotional pain. Then his experience with his Aunt and Uncle was terrible. Their relationship imploded before his very eyes, and it got to the point that Maniac couldn't stand being around them anymore. That's when he ran away. I think Maniac turns down Mars Bar because Maniac is afraid of growing attached to a family and then having it potentially taken away from him for one reason or another. If Maniac is on his own, then he never has to experience the hurt of his family being broken apart again.