As per my understanding the REST might not be available by the time the service wizard has been created in the Pega which might be the reason behind missing this option from the list and at later point of time the users have been creating service REST directly instead of going through the service wizard which might be the reason behind this option went unnoticed. I cross checked the documentation, but couldn't find any information about the REST support in this wizard. So i dropped an email to our Product Specialist and keep you posted when i hear an update from them.
If, the reason is because it has been unnoticed and there is no other specific hurdles, then I would request this to be listed in the enhancement request backlog as REST is one of the most major integration method.
We have created an enhancement request on our internal Pega Agile Studio. The ID is stated in the Relates Support Case Number field in the original post above. Please use this ID to drive this enhancement internally.