1. European laws are too restrictive. It's especially notable with stuff like the hilariously stupid online tax laws the area has (having to apply different tax rates based on the location of the user rather than the business makes it hugely more complex than it has to be) or the right to be forgotten, but it just feels we're too restrictive here in general.
2. Culture. People in the US seem more ambitious and more willing to risk everyone for the small chance of becoming a millionaire/billionaire at the end of it. People in European countries on the other hand seem to be encouraged to avoid risk and avoid anything that might lead to less job security in the short term. Probably because failure in the US is seen as a minor bump in the road, whereas failure in Europe is seen as the worst possible thing imaginable.
3. Money. Not just in the form of venture capital (though that's a huge reason), but also because US startups tend to pay their employees more, whereas their European equivalents stick to the average local wages. Hence the best programmers, designers and other such people usually either find employment in a more stable company (read, one where there's a work/life balance) or move to the US to work in Silicon Valley.
If we want a 'European Google' or European startups in general, we need to become less risk averse, more willing to reward the types of people needed to work in those companies and less obsessed about regulations.
Oh, and fix the issues certain areas have with internet connection speeds. The fact that a lot of areas in large cities like London can't get fibre internet and the likes is not helping our tech businesses compete with their US counterparts.