In Maniac Magee, why doesn't Maniac feel he can go to Mars Bar's house?

Maniac doesn’t want to go to Mars Bar’s house because he has been moving from home to home, but he realizes he belongs with the Beales. 

Maniac Magee has been homeless since he ran away from his foster parents (his aunt and uncle), except for a couple of brief stints with people who took him in.  He lived with the Beales, and then in the buffalo pen at the zoo, and then with Grayson,...

Maniac doesn’t want to go to Mars Bar’s house because he has been moving from home to home, but he realizes he belongs with the Beales. 

Maniac Magee has been homeless since he ran away from his foster parents (his aunt and uncle), except for a couple of brief stints with people who took him in.  He lived with the Beales, and then in the buffalo pen at the zoo, and then with Grayson, the old groundskeeper until he died.  After Grayson died, Maniac resumed sleeping in the buffalo pen.  This is where Mars Bar found him. 

Mars Bar tells Maniac about saving Russel from the trestle.  Maniac explains that he wasn’t able to save the boy because he was reminded of how his parents died.  The boys understand each other better after this.  Although Mars Bar and Maniac have not always seen eye to eye, this conversation helps.  Mars Bar tells Maniac that sleeping with the buffalo has not been good for him.

"Magee!"

"Yeah?"

"I had to ask you something. Now I gotta tell you something."

"What's that?"

"You smell like a buffalo."

Ears of a hundred different shapes prickled at the long, loud laughter of the boys. (Ch. 45)

Maniac may seem to not know or care what other people think of him, but he has been having a very hard time finding a home.  At this point, he just isn’t ready to go to Mars Bar’s house.  Smelling like a buffalo is a good excuse.

Having made a full circle of the zoo, they were back at the pen of the American bison. Maniac said, "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. I don't know...things happen… I can't..." (Ch. 45)

Luckily, Amanda Beale knows where to find Maniac, and tells him to come home.  The Beales are Maniac’s real family at this point.  He allowed himself to be run off from there because of the troubling race relations between the East and West End.  However, by this point, he is ready to return “home” for good.