That would be "doing very well"?
Because it seems that's what UK is doing with AZ, their production stays in the UK. And that's doing well?
If so, then EU should most definitely lock out exports of vaccines.
Sanofi announced a couple of days ago that they're at the disposal of BioNTech for vaccine production.
I'll give my answer: Because it would have been a catastrophe for many countries that depend on these vaccines - especially those that are now doing quite well. Israel, Canada and so on.
I personally find what the UK did ethically wrong. As it stands, there'll be no recourse for the EU that isn't as bad or arguably even worse.
British people patting their own back about the ingenuity of mandating exclusive contracts for UK facilities - which factually is the same as export controls - is in my eyes quite detestable.
The EU did everything wrong they could do wrong. But at the very least, they don't mandate an "EU first" clause.