When an endodontist has to do a root canal through a tooth w/ a crown, why would he tell me to get a new crown?

To prevent infection: After a root canal through a tooth with a crown, the endodontist normally will place a temporary filling. Sometimes the temporary filling can be replaced with a regular filling (either composite tooth-colored or silver amalgam). However, a new crown would have a better seal for the tooth and root canal to prevent reinfection of the root canal.

Hole in existing one: When endodontists do a root canal on a tooth with an existing crown they must drill a hole through it. If the crown is porcelain then it is common for the porcelain to crack and break during the procedure and a new crown may be required. If the structural integrity of the crown is unaffected by the root canal then you should be able to have the access cavity to the root canal filled.