Why did mike piazza become a first baseman?

Because as he got older his skills slowly deteriorated. He was not known for his fielding but he was known for his batting, he is the best hitting catcher of all time, and possess the most home runs by a major league catcher. So his fielding was not the biggest asset for the team because there were catchers who could field the positions more reliably than he could in his later years, but teams still wanted his power bat so they moved him to first in order to keep him in the starting line up, and it is the only position in the infield that i remotely related to catching and requires the least nimble movement and action.