What did u explained? The raptor engine is yet to mature. The recent explosion more or less explain it. Using RD-180 engine is due to US backward in propulsion compare to Russia in space rocket. In fact, these rocket are not cheap. The Russian charge a high price for this engine sold to US. How can they passed up this chance to milk american?did you forget? I already explained this to you earlier.
You need to thank Russia for perseverance success.
If not, NASA might need to wait for 2022 window.