The reason we didn't was critical mass. Everyone was already on linkedin and there wasn't really a reason to pick something else until the US started becoming a nuisance. It's marketing, not technical.
I'm sure an EU alternative will come up now that the US is no longer a trustworthy partner. A lot of people like myself now have ethical issues with using american products (especially from big tech) and there's a lot of demand for EU-local stuff that wasn't there before.
This is all hot air. If it's so easy to build, it would've been built by now. I bet you that there won't be a single successful European LinkedIn competitor - not for the past 20 years, not now, and not for the next 20. Europe is fundamentally at a deep state of decay at every level. The only way anything might be built, is by banning the competition. At which point you might as-well just forget about a social network for professionals entirely, because you're probably working at a gulag and there's no job hopping to be done anyways :)
I guess Americans wouldn't like to buy from Nazi Germany in 1942 and so do I with buying US-American in 2026