There were other feuds, like the Martin-Tolliver Feud in Rowan Co KY., that killed around twenty, including a US Marshall, before it finished with a shootout in Morehead in 1887. It happened around the same time as the Hatfields and McCoys were firing at each other. But it's not remembered much in comparison.

First, the Hatfield-McCoy affair was an interstate feud that grew to be a real dispute between two states. The McCoys were in Kentucky, the Hatfields in West Virginia. This meant that when Devil Anse and his son Cap killed someone in Kentucky, it was a legal challenge to get them from West Virginia. When KY Governor S.B. Buckner appointed Deputy Sheriff , Frank Phillips, to go after them with a posse, and wrote to Governor Wilson to get West Virginia to cooperate and hand over the Hatfields, he didn't get a reply, for the Hatfields were swamping the WV governor's office with petitions and affidavits that they were the aggrieved party, and that Phillips intended to bring them to KY to be killed. When WV didn't cooperate, Phillips and the McCoys then simply began raiding into WV to take Hatfields, eventually bringing 28 back to Pike County to jail. This disregard of WV predictably came to a head in 1888, when the KY posse ( containing some McCoys) was met by a WV posse ( containing some Hatfields) under a Constable Thompson. Gunfire erupted , and the Battle of Grapevine Creek occurred: no one killed, but some were wounded and WV offered rewards for Phillips and his men. In short, the police and populations of Pike Co. KY and Logan Co. WV were on opposing sides. With KY in possession of abducted Hatfields, WV filed a habeus corpus suit in federal court and it eventually made its way to the Supreme Court, which in Plyant Mahon vs Abner Justice decided there was no way provided by the US constitution for the compulsory restoration of people wrongfully abducted by people from another state. This KY vs. WV dispute is what really got the feud some national attention.

Secondly, unlike many other feuds, it went on for quite a long time. If the difficulty of Hatfields suing McCoys or McCoys prosecuting Hatfields across state lines prolonged the feud, so did the place: it was in very rough terrain, thinly populated, where raiding and hiding were not difficult, and the feudists lived there off their land. They weren't simply going to be able to easily leave the neighborhood ( and that part of WV was notably violent, with people ready to reach for their guns: the Battle of Blair Mountain happened there) . The origins are debated, but at least a couple witnesses stated it had really begun with the different families being on different sides in the Civil War , with Unionist Harmon McCoy being killed in 1865 by Confederate bushwhackers led by Anse Hatfield. And it continued, with fits and pauses , almost up to 1900. Thirty five years of articles and stories can leave an impression.

A last thing that likely played a part in the publicity and the creation of a legend was the Local Color movement in American literature of the time. There was a fad for pieces about various locales on the American landscape, and the writing tended to focus on rural landscapes and emphasize the peculiarity of the people and cultures in them. The Hatfield-McCoy feud fit right into that, as did the southern Appalachians in general. A feud in Davenport Iowa would not have gotten nearly the same attention.

Virgil Carrington Jones: The Hatfields and the McCoys