Money talks. I get that he's in a good spot in Washington, but most guys, especially guys that were head coaches in college and the NFL, want a head coaching job both for the control and the money. Maybe he pulls a Ben Johnson and stays another year, but it's a risk to take given that it's possible Washington could regress next year.
Also, Rattler, in as much as he is the TEMU Mahomes, is likely a good fit for Kingsbury's offense.