Airbus and Boeing both wanted to put off the expense of developing a brand new aircraft for as long as possible. Who ever got the new type on the market first would have a winner on their hands but the cost would have been astronomical and sales of the current type would have been affected as buyers held off and waited for the latest offering.
Incremental improvements were made to the present types but the difference was that Airbus had a much more modern basic platform which was able to accept modifications and Boeing had a 60 year old design which wasn't.