alt Hacker NewsI suppose organizers choose the easiest way to contact people rather than relying on one of the least-used messaging apps that people won't install just to attend a meetup.
Edit: It doesn't even seem to be strictly required https://news.ycombinator.com/item?id=49181030
They could easily use both, if necessary.
Or link them and more via a Matrix server - but that would require a bit more work to do.