Probably because if we buy the thing which is best for the job it will not be British, which will damage the British economy, which will in turn damage the British capability to maintain what military capability we have even further. If we buy British made products then we keep British jobs, have more British people (and corporations) paying tax, which helps to pay for the maintenence of a British military capability, dwindling though it might be.
You just cannot afford to have a military if you don't have an effective economy - unforunately our economy is massively leeched upon by the welfare state.
And before I get jumped on too hard, I am one of the people operating the sub-standard kit we have today.