When Arrec and Arlan V Durrandon both tried to take back the Riverlands why didn’t any of the lords of the Trident side with them against the Ironborn, who were way worse?

Here’s the thing tho: in the case of Arrec, there weren’t a lot of Riverlanders who were eager to have him back – maybe the Blackwoods, given their blood ties, although they would have been licking their wounds from their massacre at Blackwood Hall, and maybe the Tullys (although they were far too practical to throw good money after bad) – and a lot of Riverlanders who had participated in overthrowing him (the Brackens, the Charltons, and twenty other houses) who would have been facing treason trials if he ever retook the Riverlands. 

As for Arlan V, it’s probably a case of better the devil you know – Harwyn was a hard man when it came to tribute and homage, but if you kept out of his line of march, he generally let you alone (”their ironborn overlords had largely ignored such conflicts amongst their vassals”) – than the devil you don’t. After all, the memories of Durrandon tyranny were still fresh, and the worst of Ironborn tyranny had yet to come…

Simply put, in the Riverlands, Durrandon was not a name a name to conjur with.