Why is Maidenpool a part of the Riverlands? It is extremely far from Riverrun, declared for Aegon with the Crownlords not the Riverlords and is a extremely straight shot by road to King’s Landing but not any of the other Riverlands.

Riverrun has only been the “capital” of the Riverlands for the last three hundred years – previously, there have been “capitals” at Oldstones, the Misty Isle, Raventree Hall, Stone Hedge, somewhere near those spots where the Justmans ruled, and indeed at Maidenpool – so the political center of gravity has shifted quite a bit. 

But as to why Maidenpool is part of the Riverlands: 

• As a matter of geography, it’s on the banks of the Trident, so it would have more trade and communication with Riverlander settlements on the Trident than it would with future Crownlander settlements on the land route (especially since that road to King’s Landing is only 300 years old). 
• As a matter of politics, Maidenpool was conquered by the Justman and Teague Kings of the Riverlands and so spent a good deal of time inside the Riverlands’ polity (although it spent a good deal of time as a petty kingdom that tried and failed to conquer Crackclaw Point, and a little bit of time as a Durrandon holding during the pre-Andal invasions) and indeed served as the Teague capitol of the Riverlands.