There is no technical reason why you couldn't have a self driving bus, but the industry they are in is rather conservative when it comes to procurement. Very few early adoptors. Car buyers are less conservative, and there are more early adoptors. The cost of a "driver" is more - with a bus, the cost of the driver is effectively split between everyone on the bus, so there isn't as large a saving to be made. There is also a huge class of users who will buy a self driving car to free themselves of driving duties (parents). Hence, it makes sense to target cars first. Once the tech is working and proved, I would expect haulage firms will be quick to adopt, since wages there are a greater percentage of the shipping cost, and liability cost is higher for drivers who spend many hours on the road every day. From there it will be a short step to self driving buses, but there are other issues - we could've had self driving trains years ago, but social resistance prevented it. We might see the same attitudes holding back self driving buses.