My guess is because Discord is a better integrated communication platform in general and allows incredibly easy private messaging channels, multiple server channels and sub channels/categories, video and audio chat, better organization, etc. etc... It is just a better way to organize projects in my humble and non-expert opinion. SteemChat does not have as many capabilities. Maybe if Steemchat were more developed and beefed up then projects like this might use it more often, but right now many are using Discord to organize and develop their Steemit communities because it works so well. :)