We all know just how unstable Humankind is. This problem is not limited to Humankind only but also most of 4X games in general. I would have think that a turn-based game would have easier time to have stable MP. Can anyone explain why this is an issue?

The issue is all the data. 4 X games are rather complex. Having 4 to 10 players simultaneously generating data that must be communicated to all the other players means some things get lost or delayed, especially if one or more have barely minimum requirements meeting computers and/or questionable internet.