When you create a service that will create requests of the "ticket" request type, you have the ability to apply ticket workflows. When you change the service to create requests of any type other than "ticket", ticket workflows are no longer available for selection as ticket workflows will not work on anything other than tickets.
It may be possible in the future to have generic workflows that can be applied to any item, but workflows are currently separated out to be specific for their respective items (tickets, project requests, documents, etc.).
I hope this helps! Please let me know if you have any further questions!