It seems like a mix of different things.
Some of the people who always participate didn't this time, maybe because we're going too far in the past and it gets harder to remember how much you liked one game over an other? I know I struggled with that too.
But 2010 was one of our record years in terms of participation so it might just be loss of interest like you said.
It's also a bit unlucky: with the current amount of voters we could have ended up with a top 25 if the votes were spread differently, but this ended up being a very "top-heavy" year where a small group of games got a lot of points, leaving only crumbs to the rest, that's why we've got tons of games at 5 points or less. That's just the nature of the game and means extra praise for the games that made it.
I'm sure when we do 2020 it'll bounce back :)