I don't have a fully formed theory, but I believe it is a combination of things:
* there aren't any real EU tech giants
* the regulatory environment is tough on businesses: it's hard to start one and follow all the rules. The rules can also be different across EU states.
* poorer local market. Europeans generally don't have as much disposable income as Americans. It's harder to get a good amount of people to pay 20 EUR / month for a SaaS.
* different investor culture: if you failed once, you'll always fail thinking
* corporate and personal taxes are high, so there's not a huge difference between making 50k and 80k pre-tax.
* overall tougher social mobility, making it hard for top talent to get to the right people for funding, ideas, mentoring, etc
The lack of large and successful pure tech companies, means there are a bunch of good engineers, but little competition for them, which means lower salaries.
Some people say healthcare and more holiday days are another reason salaries are lower, but I don't believe that. First off, 2 more weeks of holiday doesn't add up to a 100k pay cut, neither does paying for private insurance. Second, it's nice that there's a govt run fallback option, but anyone that has had to deal with the system (in most, but not all EU states) knows that you eventually end up having to pay out of pocket or take private insurance for anything more serious anyway.