Hulu and Disney Plus have taken centuries in this endeavor. There's a lot of content licensed to Hulu that is not necessarily licensed to Disney Plus, though Disney Plus seems to be showing more Hulu content, but I assume it has to do with licensing.
I assume it has as much to do with the fact that Disney and Comcast didn't settle on terms of sale for Comcast's minority stake in Hulu until June of this year.