(spoilers main) Is there any reason beyond plot convenience that Robb and Myrcella wasn't betrothed?

Seriously, Robert was so in manly love with Ned that he wanted to marry a female copy of him, with Rhaegar kidnapping and raping Lyanna that failed but when the chance arises he wants to unite their houses with a marriage and he is so super into it and yet there is only a Joff and Sansa betrothal when Robert has not just one but two other children and Ned's heir who was almost an adult isn't even betrothed and Myrcella Really surprising Robert didn't arrange a marriage between that two as well and didn't arrange Tommen who was old enough to become Ned's page or perhaps even leaving him in the North to be fostered there.