I know why hockey romance is popular. I'm a sicko hockey fan, and I know what most of these players are about and why women find them attractive.
About 4/5 of them are at least 6 feet tall, and they pretty much all have to be in top physical shape with no extra flab that football players either can use (those big fellas on the line) or can get away with.
Future pro hockey players get pulled into the system when they are 14, 15, and get their education on the road and/or at home. There's little time to pursue any interests outside of the game, and generally they don't read, or follow current events or anything of the sort. So, they are simple folk, and their partners will typically be able to dominate them in any argument or debate. Hot dude with a 6-pack that you can talk into a corner? Oh baby.
One of the qualities that female readers must love (this is me commenting having never read a hockey romance) is the whole loyalty thing we've got in hockey culture. You don't let your boys down, you keep yourself honest and accountable to your play, and you own your mistakes. The hockey boys stereotype is that they have integrity, and for the most part that does seem to hold up.
Oh and also they are millionaires, so there's that too.