Simple thing, yes as stated in the article, European VCs are very picky and strict about who to fund and also have a lot less cash, so they just invest in fewer, more carefully picked ventures, so their success rate is higher.

It's like a typical business that works well while it's small and only captures the best corner of a market but has it's returns deteriorate once it tries to serve virtually everyone.

Trick is to get same or better returns while having same or better deal volume, EU is far away from that.

Another factor might be that, due to visa restrictions, many great European entrepreneurs have decided no to move to the US.

EU has exactly the same problem. It's hard to move into unless you want to ride an inflatable boat.

Not really ? There are clear routes to a visa (France, etc) for degree educated tech workers.

Well, there are but only if you wish to work for a salary somewhere. If you want to be self-employed or do your own thing, it is not possible in some countries and will pull at least a couple thousands euros per month in mandatory minimal taxes in others. Sometimes 3-4 thousands.