2 word answer......French protectionism.
Now if HM Govt had invested in the BAC 3-11 project, the UK would now be world leaders in civil aircraft design and supply. Airbus certainly would not have existed. Sadly, and this has been well documented elsewhere, it was Concorde or bust. Airbus soaked up most of the design and build expertise caused by the implosion of our civil airliner business.
The A300 was originally a design concept by none other than the UK aerospace industry and from memory, so was the A320? HM Govt had to go begging to enter the Airbus consortium after the failure of Concorde and did so under French terms.
France and Germany will do whatever it takes to retain its own workforce. After Bae sold off its share of Airbus, wing production within the UK was always going to walk a tightrope. Let us hope history does not repeat itself once again?