Protection

I don't think you're right to say more people crashed in 2009.

Yes, that was only a superficial impression--I didn't mean to say that it was actually the case. It's probably just that I haven't watched much polo for a while, and lack the experience of playing it.

The bike-handling skills, & spatial awareness has improved immeasurably since then.

That is obviously going to be one of the more important factors, and easily capable of offsetting more minor trends. I think what may have skewed my perception was that I couldn't remember as many unforced crashes from two years ago, e.g. when people tried to turn tight corners fast and then crashed without anyone else being near, but perhaps that's only because they surprised me more and therefore seemed more frequent than they actually were.