On the front page it states that the Pirate Party is a movement based around civil and digital liberties, privacy and anonymity, and transparency. Using a proprietary service for party communications that's based in the USA and that has already had at least one user data breach, when there are other, better options available, runs clearly counter to those principles.
(Also the programmer in me takes one look at Electron being used and struggles not to vomit. But that's a different issue.)
If someone suggested moving all communications to Facebook I expect there'd be even more controversy. As far as I can tell having a presence there is more of an advertising necessity, and feeding the giant privacy-violating behemoth beyond that purpose is probably not something we should be doing.