It may just be that ROI doesn't make sense: very few user out there truly care about (or even understand) E2EE, for quite some users it creates an inconvenience & support incidents (harder to move from device to device, forgot your passphrase - lost your history, new joiners to a group chat don't see previous history, etc), it requires a significant additional engineering effort to just maintain it, many new features get shipped much slower because of it...