Maybe the problem is in the system of government. Secretaries of State really have to be MPs, though I do understand that occasionally peers will do, though even they, it appears, will be fully elected in the future. Being an elected official, they have to make themselves electable.
Rather than being focussed from the outset on matters military, they have to be jacks of all trades, with the attendant lack of mastery. Furthermore, aspirant MPs probably have the aim of the Premiership, not a lower Cabinet office. So Defence is not their primary aim nor consideration. I don't suppose Margaret Beckett grew up saying "I want to be Foreign Secretary one day".
For continuity, we have the senior Civil Servants; though these people do not appear to be remotely connected to the coalface. Oddly, this is in contrast to MPs, who will in fact go on a jolly every now and then to the less dangerous parts of Aunty Betty's train set. They have to make themselves appear electable, don't they?