How did the greyjoys become lords of the iron islands ? Harlaws should be the richest and drumms or goodbrothers the strongest

Lordsport being the largest town in the Iron Islands suggests that the island of Pyke is one of the most populated, and probably has a higher level of GDP per capita, between the harbor’s trade and the skill of its smiths. So it’s not that far behind the Harlaws’ in wealth and population. 

As for why not the Goodbrothers, I imagine it’s because Great Wyk is so large that they’re not totally dominant on the island, since they have to contend with the Merlyns, the Sparrs, and the Farwynds. 

Also, the Greyjoys were elected to be Lords Paramount in 2AC.