Because they had to
I wonder how much of the A380 was because Airbus had no competitor to the 747 ?
If a national airline wanted long haul trans atlantic/pacific it had to buy 747s (before etops 370). So you had to let the Boeing salesman in the door, at that point he could offer you a much better deal if you also wanted to buy 737s and 777 for your other routes. And look at all the savings on maintenance and training by only operating Boeing.
It's like HP or IBM having to buy up auxiliary systems vendors because if they can't offer the full package the customer has to go to your competitor