I think the answer is quite simple.
A 50cc scooter can travel at around 30 mph give or take. A (very) fit cyclist can hit those speeds, even a fair bit faster downhill.
To ride one you need training, a licence and insurance. The other you can just jump on one day and go for it on any major A road you fancy with no clue of how to be safe.
I think the time has come to make some of licencing for cyclists compulsory. If; once you pass, you jump a red light its 3 points and a fine. Simple!
Insurance would obviously a touchy subject but if governed well shouldn't cost more than £20/30 a year, if you as a cyclist injure someone, or damage someone's property i.e. lash out and kick a car you will have liability cover for the third party to claim from. These policies often cover theft from anywhere too.