Same reason Hulk was. Coaster track has a certain lifespan and every 20 years is the recommended timeframe for replacing track that runs a minimum of 12 hours per day 365 per year. That window is closing for Mummy.
Then again, Space Mountain should’ve been retracked at least 15 years ago if not more and they keep skating by with it so it’s merely a guideline. Safety issues can arise if you run too far past the recommended timeframe, which is getting to be the case for SM.