It's the wrong question.
The question should be "why would they want to?".
Signal solves a specific issue (private messaging), and decentralisation does not help with that at all. As far as i can tell it makes it harder.
Therefore there is no need to be compatible with a decentralised service like delta which solves a different need.
@newhinton @signalapp I guess the reason would be to allow deltachat users to communicate with signal users, which would be very good for deltachat users and I cannot see why it would be bad for signal and/or signal users.
It would also make signal less of a walled garden if someone no longer trusts, or disagrees with, the signal foundation.
Note: I do not use deltachat (yet?) and do not know too much about it. What need does it solve, if not private messaging?