Any information on why the companies can't work together?

> Abandoning the project will be a blow to efforts by European countries to cooperate more closely on defense and to present a united front as they contend with a hostile Russia and souring ties with the United States.

Nonsense. At most, you don't get a single monopolistic, design-by-committee offering being forced-fed to the EU. You will have instead Dassault Aviation presenting their offering, and Airbus can work independently on its own proposal.

Sounds like win-win to me.