why didnt scrooge paint over marleys name on the sign that hung outside of their place of buisness?

The sign didn't matter. It was the name of the business, and as long as business came to his door..... he would answer to either name.

Scrooge never painted out Old Marley's name. There it stood, years afterwards, above the ware-house door: Scrooge and Marley. The firm was known as Scrooge and Marley. Sometimes people new to the business called Scrooge Scrooge, and sometimes Marley, but he answered to both names. It was all the same to him.