1. It got better received than author expected (or he got more of it) so the author started to put more effort into what was originally supposed to be low-effort fun posts
2. Another author or assistant liked it and offered to help. So Hata draws, someone else enhances it
3. Hata actually started to work on this years ago, and as time went on, the art style changed
Simple combination of COVID-19 and nothing better to do.
He probably have a full studio at home. Maybe he just drew the first with just iPad Pro. I don't really know how his stuff work since I can't draw for shit tho.