I expect the answer is money. However, I do detect an institutional dislike to 'MSAM'. In Germany in Cold War and now for deployed ops, we opt to rely on allies to provide this capability (including its BMD element) in return for UK capabilities in other areas.
Amongst close allies with such capability are NLD, DEU, FRA, and ITA. Somehow they all see need, while we don't.
Regards
Batco