In Chapter 14, Dill tells Scout why he believes Boo Radley doesn't run off.
"Maybe he doesn't have anywhere to run off to..." (Chapter 14)
For much the same reason Boo Radley doesn't leave town, Mayella has no where else to go and no money with which to establish herself. She has no friends, and there is no mention of other Ewell relatives who could take her in. She probably feels responsible for her seven brothers and sisters who would be left on their own to deal with their father, Bob. Most importantly, she probably fears her father more than than anything else. Bob has apparently beaten her before, and she probably worries that if he caught her leaving or found her later, the punishment would be worse than ever. She leads a terrible, lonely life, but running away may have even been worse. Perhaps a better question would be how Mayella's life turned out after Bob's death. She was left on her own with no finances except for, hopefully, a continuance of Bob's welfare check. Mayella may have had to go to work, leaving the other Ewell kids to fend for themselves. Any of these scenarios leave Mayella in dire straights, living a life of extreme poverty with no relief in sight.