Couldn’t Robb have just sent ravens north ordering lords to gather and re-take Moat Cailin from the north? The Mandelys alone could probably have done so.

Robb could have sent the ravens, but the question is who would have gotten them? 

Winterfell had fallen, so there’s no Stark to coordinate the reconquest; the Northern lords are thoroughly distracted by the Hornwood Crisis and even when they get past that, the failed attempt to retake Winterfell further scatters the loyalist forces.

These are all dominos that have to fall for the North to fall, because as we see in ADWD, even with everything going against them, the Northmen still put Moat Cailin under siege, with the crannogmen attacking from the south and the Ryswells and Dustins from the North, and then Ramsay and Roose coordinating from north and south of the Moat. If Winterfell had not fallen or had been recaptured, Moat Cailin would no doubt have fallen faster.