Why did they stop using the P-51D's?

Think about it... We have high powered jets that can travel 2,275 Mph.

Mainly, the P-51D became outdated when the anti-aircraft technology increased, posing a greater risk to people flying the P-51. It retired on January 15th, 1961. The last combat use of the P-51 was in the Isreali Air Force, totaling 79 mustangs.