Elena blood is not longer useful to create hybrids, because she is no longer alive (or human). Hybrids are part vampire, so they need human blood, albeit from a doppelganger one, to complete the transition (just like regular vampire need human blood to turn). As for the travelers, even though Elena and Stefan are vampire, in my opinion they are still consider as "doppelganger" because i think being a doppelganger is like a supernatural status, not a species. Thus someone can be a vampire by species, and a doppelganger by "status".
Or maybe the writers are just lazy and didn't see this plot hole coming... ^^