Probably because different messaging platforms have different opinions on how to implement encryption, and those opinions are baked into their infrastructure at a pretty low level. If two platforms don't support a common encryption system, the only way to move traffic between them is to decrypt and re-encrypt the data at the boundary between platforms, giving both platforms access to the unencrypted messages.
Mandating a common system for E2EE seems like a good step 2, but just getting them to exchange messages at all is a good first step that doesn't require anyone to change their backend to support a different encryption mechanism.
(Just to give an example I'm familiar with, you can tell Facebook's encryption isn't E2E because you access Facebook Messenger from a new device and have access to all your old chat history. Making Messenger support E2EE would break a basic assumption about how it works and what features it offers.)
The dude looks like Michael Jackson to me.