The European market versus the American market. They tracked and followed a similar path until 2010, but have diverged since then. 

My guess what happened was that there weren’t many important tech companies and the U.S. has produced most of the important tech companies. 

Any other theories of what happened?

Europe too focused on regulation and bureaucracy while so much of its talent left for the US, it historically suffers from a huge brain drain which stifled its growth potential

I'm only an armchair economist, but my instinct is that it relates to the Fed lowering interest rates earlier and lower than the ECB. That allowed US companies to take on free debt.

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