I guess the irony of it all is that all those boots on the ground couldn't secure Iraq.
I'd turn it round and ask why Iraq couldn't be secured with all that air support?
In financial terms, the RAF is terrible 'value for money' when it comes to military capability. If the government come up with an intent to go somewhere and do something, most of the proposed actions will be cheaper when undertaken in the main by Land forces. They can also hold an area once taken - rather hard to do that with air assets over a 24/7 duration.