Domestic chart only uses Bugs, (Korean)Spotify, Flo, Melon, Genie - and now apple music.
Global Kpop chart includes all those + Spotify and YT I believe.
As for why they don't include korean Youtube, perhaps their not confident in it's ability to correctly confine it's chart data to South Korea only, or perhaps they want additional things to certify domestic, such as ULs (which the video from the Gaon Vice President makes clear they have, even if they don't include it themselves).
Another factor is YT music is by far the lowest revenue share per stream, and perhaps that's a point of contention, as GAON is a certified body for revenue distribution.