Because it’s only a recent addition to the Olympics and the entire 7s circuit has been thrown off kilter by COVID. The sports itself needs to find its feet again. If participation drops in the next few years and they don’t get back to the hype levels of 5 years ago there is no guarantee they’ll keep it as an Olympic sport.
I think it's unlikely it will be dropped. Rugby as a whole is growing strongly and most international sport has been thrown into chaos with COVID.