Its absolutely ridiculous that Twitter does not have end to end encryption of DMs yet. To think that they once hired Moxie/Whisper Systems and could have been miles ahead of everyone else on this. It's purely negligence at this stage.
Twitter is fundamentally a web app. Users can log in from any browser and read their messages, which are stored on the server. This is a very different situation from Signal or WhatsApp, where an account is tied to a device, and messages can be stored there.