No one has answered with one of the biggest issues with the shuttle: each one was extremely custom. Every single heat shield tile was unique to a specific position and a specific shuttle. There were probably over a hundred million individual components in the Shuttle, and with many critical ones being custom, the time to refurbish it for a new launch was much longer.

This is in contrast to something like the falcon, which has a very standardized mfg process and components, which allows for really rapid iteration