You may have guessed it right!
Because Of most of the people have their dominant hand as the right hand.
But There Is More to it,
See This Gear-Less Scooter,
Now Take a scooter looks from the side :
You Can see Everything on Left Side.
There is another reason to why the left handle is a dummy.
When you try to mount a scooter on its stand, you grab the left handle, your stand gets engaged from the left side.
In doing so, you actually apply torque on the handle.
We know, Not a good idea to apply torque on the accelerator if there’s no one on the bike.
The left handle is purposely made non-functional so that no one actually does something horrible while engaging a scooter stand.
Amazing how much thought goes into designing something that looks so simple.