This appears to be a bus from a particular Park & Ride in Fife that's not very used to Edinburgh Park, which is quite remote from the centre of town. So it's possible it's just not a very popular route. There are people who work near Edinburgh Park, but I'm not aware of it being a massive destination.

And yes, the driverless bus was clearly more for testing than it was for actually carrying people. I'm not sure who is doing the testing, but they seem to be significantly behind other driverless vehicle organisations.