Just a guess, but maybe it's just a good starting point. 30kmh is pretty quick for moving goods (if you factor in all the downtime that goods spend in warehouses) and is also not terribly hard to deal with in terms of designing the system and the traction engines. When you combine this with the fact that the density can be higher than with human drivers, 30kmh doesn't seem so bad.

Plus I would think they could gradually increase the speed as the system proves itself more. There are so many other challenges with getting this done, that maybe making the system go as fast as possible took a backburner.