Why do the number of playable bets differ so much?

Generally, the closer to an event it is, the more bets appear. This is due to the fact that sportsbooks tend to adjust their odds more frequently closer to a match (depending on lots of factors such as the weather, injuries, team/player form, rumors and so on). Some bookmakers also adjust odds in order to attract more customers closer to an event (to even out their risk, they always want about the same monetary risk on all outcomes).

The number of bets available therefore depends on how many, and which sportsbook accounts you have available, what the sport is in the sure bet, season (what events are running), day of the week and even time of the day. Most often there are more bets at the end of the week (Thursday-Sunday) because more matches are played at the weekend.