I think that you are an unknown man claiming to be king, whose House broke faith with the Targaryens not so long ago. After so many attempts to her life, would Daenerys Targaryen trust such a man if she were far from home and without an army?

[Fine, let him have his way. She'll lower her pretense, but not completely.]

From what I heard, the last Westerosi to approach her for an alliance offered marriage and an army. The Dornish, however, were as insulted by the war as the Targaryens.