Austen makes it quite clear. he doesn't want to settle Mary in his house because he does nto wnat to be tied down to staying there himself. He is willling to stay at Mansfield for a while because he's amused by the Bertram girls. He knows htat Mary woudl not be content to live at Everingham for long periods on her own, she's not used to coutnry life or to being away from London society... and he's too selfish ot make evne a minimal adjustment to his way of life to please her...