a) Having English be the national language is a massive advantage which only Ireland shares (and Ireland has plenty of American tech outposts). That's an advantage people don't mention often.
b) The EU keeps adding regulations, which make it a less attractive place to invest. The UK seems to copy each of these regulations though, which maybe makes this a moot point.
But I think ultimately, it's probably down to network effects. The same way Twitter/X stuck around even though basically everything about it changed, cities might do the same.