The same reason we got rid of Sea Dart on the Invincibles. All the deck space, crew, magazine space, power etc etc you use for a missile system isn't available for supporting aircraft.
That and money
It’s always difficult evaluating missile systems because it often depends on what mod you are talking about. The latest version of RIM-116 entered service in 2015 and is much more capable than the version tested by the RN. Not to say that is the appropriate system for the RN, only that the previous tests may not still be valid.
Crowsnest on its way out already, whether it's get transferred to the next gen rotary wing platform or a new system is developed I don't knowA recent release from the UK’s Defence and Security Accelerator (DASA) has said the UK will retire its troubled Crowsnest Airborne Early Warning (AEW) system, which has only just entered initial service, before the end of the decade.www.naval-technology.com