I think the VC one is the main factor here, as is how risk adverse many investors seem to be in Europe. Put simply, the majority of them don't like investing in B2C companies, they don't like investing much money in general (by US standards) and they certainly don't like investing in businesses where it'll take years for it to pay off if they're lucky.

This means that if you ask for investment in something like Facebook/Twitter/Instagram/YouTube/Reddit/whatever you're almost certain to be turned down by European investors, because they're seen as too risky compared to say, a shop saying office software to ad agencies. But these high risk businesses are also ones with a high pay off if they do well.

There's also the fact European investors seem to have a relatively 'stingy' standard when it comes to how much money is worth investing in a startup. While over there in the US it seems like unproven companies are getting tens of millions of dollars on a whim, over here it seems like £50-100K is seen as a 'reasonable' investment amount, with some offering even less than that.

That money is not gonna be enough to hire Google level software engineers, have all the fancy buildings and facilities that many young folks expect, pay for high end hosting platforms, pay for a massive marketing/ad campaign, etc.

So even if you did get funding, you'd likely be outcompeted by US based companies flush with cash and resources that have years of runway compared to your 6 months or so worth.