It's proven, in production, and in service with the UK's two closest allies. Australia got a hold their 24 quickly with a minimum of fuss and no delays or in service headaches so the RN could do the same and have the expericence of an air force that operates very much like the RAF/USAF/USN on hand for easy exchange postings. Also the reason governments like the most: it's cheaper.
Edited once to fix rubbish spelling.