Actually, nobody did. It seems highly likely that its a misconstruction of Anders Ericsson's research findings that expert-level performance of the violin required 10,000 hours of practice: he also studied top level athletes and chess grand-masters and gave them the same 10,000 hours to expert level performance.
The 10,000 hours rule was then snatched up and mis-applied to all sorts of new projects that an individual might take up, and trading is just one of these. Ericsson never studied trading.
Have a look at these for a bit more detail -
http://www.bbc.co.uk/news/magazine-26384712
Let's face it, the rule has no evidence so its just another trader myth.
The 10,000 hours rule was then snatched up and mis-applied to all sorts of new projects that an individual might take up, and trading is just one of these. Ericsson never studied trading.
Have a look at these for a bit more detail -
http://www.bbc.co.uk/news/magazine-26384712
Let's face it, the rule has no evidence so its just another trader myth.