1. Many English people have time off from work and can go to the games
2. English people are spending more money because of their Holiday spirit, and thus people whom are typically too frugal to go to the game might just find it within themselves to sit in the cold and watch their team in person
3. Globally more people will watch the games because their leagues have taken a break
I would think that the players from the other leagues would be fresher because of this break, and therefore excel in competitions like the CL and the UC, yet that is clearly not the case. If I had to venture a guess as to why not, I would bet the money raked in by English clubs during the Holiday season is enough to add quality depth to their squads and therefore offset the rest the players from other leagues receive.
On a separate yet related note, since most people are just dying to fill the void left by the absence of the other leagues, I propose that the MLS play a mini tournament during this time. For the reasons listed above it is quite possible people might actually watch it.