after i was hired, why did the doctor do this?

after being hired at my current job, the doctor let go of the ma he had working for them before i came....

when i was looking for employment, i called this doctor twice... the first time he said no then second time i called he said to meet him at boa.. i meet him in jeans at boa and gave him my resume, references, and recommendations. he then said, he will think about it.....

the day after martin luther king day, he sent me a text message asking about my job and i stated i was still looking then he said to go to the office and meet him...

when i went there, i met the rn who works there and she stated the doctor liked me...

we talked and the doctor said to start on january 23rd...

a few days later, the doctor told me he let go of the ma that was working there before me because of me....

a few weeks ago, i showed a resume to the doctor and he said, "we have you and we don't need anyone else."

why would a doctor let go of an ma and hire me if he already had someone working?