As someone who has implemented operational transformation, I believe Google dropped it because it consumes a lot of network resources. If it became popular, Google would have to dedicate a fair chunk of infrastructure to it. Could they do it? Yes. But I believe they ran a little cost/benefit analysis on it at scale and probably decided they didn't want to continue with the wave model.

I am aware that some operational transformation is used in docs, however the usage scenario is different: collaboration is not the default use case and the number of collaborators would be smaller than in a wave, where the default use case is many waves with many participants.