Why do you think did Asha, in her Kingsmoot speech, suggest the idea of the Ironborn acquiring parts of the Stoney Shore/Sea Dragon Point from the Starks, instead of lands on Cape Kraken/Flints Finger?

Because Asha still held Deepwood Motte, which is the main seat of power in the Sea Dragon Point region, and Flint’s Finger was still held by the North, and while Asha had an agreement with Lady Glover, she didn’t have one with the Flints.