With the destruction of King's Landing, most of the lords were unwilling to follow a Lannister child King with dubious origins that would rule from the Westerlands. Nor were they willing to follow a Targaryen pretender again, especially after that line had produced one too many cruel madmen. More than one Lord would love to rule themselves again, separate from the affairs of the other Kingdoms. But they were also unwilling to declare a green, unproven, and lusty summer boy like Harrold king.