The technologies are orthogonal, but I do think it does make sense to pursue both of these things together. EVs are much more energy efficient. They will cost less energy to run. The choice to go to EVs also makes sense if you think about robotaxis, as EVs will require much less maintenance, they have much fewer moving parts.
You also have the environmental argument to consider. Gasoline-powered Uber rides are not environmentally friendly. The general assumption is that robotaxis will decrease the price-per-ride to the point where there will be a big shift towards less people owning cars, and probably also less people using mass transit. That will mean less people taking buses and more people in cars. If all of your robotaxis are EVs, this is less of a concern.