The shuttle's main engines were extremely high-performance. They were marvels of engineering but required a lot of work between flights. Just like cars can be optimized for performance or reliability (think Honda Civic versus Ferrari) rocket engines can be made more maintainable and tolerant to long use periods. Not saying Spacex engines are Hondas, but they are probably way less trouble than SSMEs.
Soyez has been flying nearly 50 years. It's the dump truck of space, with 1700 launches to date. Compare that to the much more elaborate Shuttle's 135. Elon would rather have the dump truck than the Ferrari.
