Because investors in Europe are a bunch of pathetic spineless banker types.
Or investors in Europe have enough opportunities to generate more than the typical return on investment of VC? The average VC ROI is really low, it gets outperformed even by passive index funds IIRC.
Edit: Found the source.
"Yet 2013 annual industry performance data from Cambridge Associates shows that venture capital continues to underperform the S&P 500, NASDAQ and Russell 2000."
https://hbr.org/2014/08/venture-capitalists-get-paid-well-to...
The top companies only want to go with the top investors and vice versa. I wish they'd publish data. I'd love to see Benchmarks ROI on their recent fund, it's likely going to be the best performing fund of all time.