Most likely because with Brexit the UK removed itself from science cooperation with the EU (not sure if they've arranged an alternative), so a scientist current

Most likely because with Brexit the UK removed itself from science cooperation with the EU (not sure if they've arranged an alternative), so a scientist currently working in a UK institution probably aren't getting invited to work on projects with EU scientific institutions. In the EU, the institutions getting EU funding are probably free to engage EU suppliers (for products as well as for know-how, i.e. to talk to/work with professors in EU universities), but if they want to engage a non-EU partner, that's a different story... If you're a prize-winning scientist, any country will probably love to have you, and you'd choose a location where it's easiest to cooperate with others...