Given the retention issue the RN is having, giving up a gym is a serious negative consideration. I know some people think navy personnel will put up with anything, but that really isn't true, and on long deployments to places like the middle east, an enclosed, spacious airconditioned gym can be the most popular place on the entire ship. It serves a very practical mental, physical function for the crew, and that large flexible space can also be useful for other purposes on occasion as well.
The Mk41 are much heavier (AFAIK) and larger, so it could eat into future growth capability. Also the type 26 are going to have the strike capability, leaving the T45 to focus on its air defence role. The Mk 41 would have likely had less local content and cost more. If they were just to be armed with CAMM, a bit of a waste. Money can be better spent, else where, there are plenty of funding holes in the RN.
IMO I think it would have been neat to have those Mk41 strike length tubes. But getting camm is a pretty nice upgrade to an already capable platform, particularly some of the other issues are resolved. The RN Is probably thinking about planning for their replacement, so gilding them up also may be a bit counter on that front.