I was about to say the same thing. I'm a huge test cricket fan (only form of cricket I really like to watch). For me part of the appeal of Test Cricket (6 hours a day for 5 days) is that the Australia summer is too damn hot to do anything else. The big match (Boxing Day Test) coincides nicely with the break week many jobs have between Xmas and New Year. So the fact that you have a bunch of downtime from work combined with it being too damn hot to want to do much else makes vegging out on couch with a beer (or several) and the cricket on the box all that more appealing.
I do not know a lot about American football but I recall hearing or reading somewhere that the breaks in play make it a more popular spectator sport. As it allows for discussion and analysis of the prior event in a way more freeflowing sports do not.