Wasn't it because litterally everone else who had a better claim to the throne had murdered eachother by that point?
Which is why you would get the throne fracturing as opposed to unifying under Bran.
I mean he has zero claim to the loyalty or fealty of Daenerys's forces and the fact that his "brother" killed her makes it unlikely that they would follow.
He doesn't actually have claim to the North, Jon was proclaimed its king, he bowed to Dany (before killing her), and Sansa was named his heir. Nor does Bran have any real claim to the armies of the North; if he wants Northern loyalty then it would be as the prophet of the Old Gods and a Stark far more than anything of his own.
The Faith wouldn't just oppose him, they would flatly call him a heretical, sorcerous, abomination spawned from a line of such.
The Lannisters are largely broken and while Tyrion is still around, he lost the support of Dany's forces and has zero loyalty to him in the Westerlands; nor does Bran really have any claim to his support.
If the Tyrells support Bran then they will face open civil war as the Hightowers take the opportunity to move against the stewards when said stewards are already weakened and without allies; it would also be a religious war.
Bran might keep the Riverlands and Vale thanks to his blood connection and Sansa, but both are also Andal heavy and the Riverlands are massively ravaged by war. Neither would actually support him using military force to secure the throne.
The only two people who could have sat the Iron Throne by acclamation and with broad acceptance were Dany and Jon. Of course, Jon had to be an insane idiot who proved that his fathers (both of them) political idiocy bred true.
Granted, Dany deciding that she had to conquer the entire world really did come out of the blue and was flatly insane.
