Perhaps some anti-USA attitudes contributed? I would have put Napoleon first for the criteria and because he represented England's foe France, the great enemy for the longest period of history.

Here's why Hitler was not on the list:

To qualify, each commander had to come from the 17th century onwards – the period covered by the museum's collection – and had to have led an army in the field against the British, thus excluding political enemies, like Adolf Hitler.

And here's probably why Michael Collins topped Napoleon:

The online poll was launched in the middle of February, and around St Patrick's Day – March 17 – there was a surge in support for Michael Collins, although several people pointed out on the museum's website that, technically, the guerrilla leader never led an army on a battlefield.

I guess that when an opponent beats you (Washington), he is naturally going to climb the ranks as a “foe”.  However, I would argue that Napoleon was a greater “foe” in the sense that he was a more capable general, in command of a more dangerous army.

Napoleon was more capable in a tactical and operational sense but was a complete failure at designing and implementing a winning strategy or Grand Strategy if you will.  I actually think the list is pretty good.  Washington proved to be somebody the British just could not beat regardless of the outside help he received.  The Continental Army Washington built was s resilient it managed to rise u from the ashes several times during the war.  Washington was also instrumental in developing and championing the strategy of combined guerrilla and regular warfare that eventually wore the British down to the point they decided that holding onto the colonies was not worth the costs they were incurring.