Western Pennsylvania was more or less where the junction of these three populations and settlements was located in the 1750s.  French fur traders and trappers had settled along the Great Lakes, the Hudson River and Quebec, while the British had settled west to the Ohio Country and elsewhere in Canada.  Natives, of course, had been there for generations, and suddenly found themselves on the knife edge between two competing empires.

The British and French both tried, with mixed success, to play the native tribes against one side or the other.  Often if an enemy tribe had allied with the British, then the French might be able to convince a native tribe to ally with them.  If the British had driven one tribe from traditional lands, the French promised them aid to recover them in exchnage for alliance and scouting.

Western Pennsylvania was simply the geographic region where these three peoples would naturally clash, based on where they had settled and what their ambitions were.