Difficulty. Humans and argonians never get along. The Hist do not like the descendants of the ehlnofey very much.
Indeed, a source tells of the knahaten flu being manipulated into form by the use of the Hist's spores, from an argonian shaman.
And a seperate aspect of The Hist directly helped Umbriel wipe the many foreign influences in Black Marsh.
So unless the Hist or argonians were being directly threatened by some kind of arcane anti-matter bomb from the Thalmor,
the Ann-Xileel will not intervene. However if the Thalmor managed to cut their way through Cyrodiil and started culling populations
of Black Marsh, there will definitely be contact to Skyrim and any other remaining human kingdoms. Or maybe they'd just beat them
back enough (like they did with Dagon's forces) to allow the human nations a chance to make a precision strike at the Thalmor.