Every IRC community I used to be a part of even a year ago has made the jump. It's just more convenient for all concerned- the only holdouts are people clinging to their CLIs and people objecting for ideological reasons.
I wish it hadn't come to this, but IRC's just stagnated for too long. The barrier to entry's too high, using it on mobile devices is a chore, the feature set's extremely limited, etc. Had IRCv3 managed to get off the ground and made some strides towards creating a modern user experience maybe two years ago, Discord wouldn't have had such a vulnerable market open for it to stroll in and snatch up.
Discord provides seamless cloud message logging, mobile notifications, intuitive clients, a huge ecosystem of easy-to-use bots, and integration with games that just works without the user having to set anything up. Even non-gaming-related communities have been switching to it for convenience's sake.