As I think we all know.... There are two primary reasons for these numbers (slots vs. sports)
1) The anonymity of playing slots. Just any dunce can walk up and function on a slot machine. People are all by yourselves and don't feel awkward not knowing what they are doing, so therefore many more people involved and getting fleeced.
2) Why would books want to take too many serious wagers on sports? The very rare individual can win from them in this activity. Slots are pre-programmed to kill ALL players.