It's my understanding that the Basque language is from a totally separate language tree from its neighbors. While that may be an anthropological or linguistic focus to the question, was there any reason why this group of people remained isolated for so long from conquest, whether it be some sort if terrain, no interesting resources, etc. Or any combination therein.
Another way to put it, language shifts depending on so many factors, migration and war being two; but the Basque language is distinct from the people's who lived all around them for centuries. Is there some historical reason for this?
My best guess is that before Romans came into the area, basque language was one ammong a lot of non indo-european languages. But i doubt geografically, basque language was where nowadays Basque Country is. It was only used by people in small towns in deep Pyrenees. As you can guess very isolated place.
With Roman empire latin got standarized even in nowadays Basque Country territory. After that, by 6th century these groups from Pyrenees came to what is nowdays Basque territory and got people to use it as their own.
Like you guessed, difficulty of therain and small amount of natural (except metal) resources made it not worth it to fully conquer, so it survived until now.
Sry my broken English ;)