The term "mountain course" is much harder to define than "links course". Banff, for example, is undeniably set among the mountains, but the site is pretty flat as far as I can discern without having been there myself. 
Since I haven't been to Banff and Jasper, probably the highest-rated "mountain courses" in The Confidential Guide were Cape Breton Highlands (though I never would have thought that was a mountain course until it was named above) and the Upper Cascades at The Homestead ... they got 7's or maybe an 8 on the Doak scale. Ekwanok is also very cool.
It is not impossible to build a 10 in the mountains, but remember there have been way fewer attempts at it than in other locales, and there is usually a lot of terrain which has to be overcome at some point.
Of course few attempts [comparatively] have been made thus far. It is the natural progression of golf.
Golf has been played by those who could! Therefor, courses were built near those. Since most wealth started along ocean/sea/river access, the courses were there. Then, as progressive economics brought wealth to the plains and rolling valleys of the countryside, golf developed to follow. Then in the 1990's money in the USA began to reach even Appalachia and even Bobby Joe could afford a cheap set of clubs, so the mountains became a possible site. Of course, it was 1st going to be downscale.
Tom, you really missed the point on the terrain. It is not there to 'overcome', it is there for the imaginative mind to use as a unique opportunity for new insights. Now, it is true you cannot re-create St Andrews there ...... but so what? We already have one!
My beloved Eagle Ridge is not nor ever will be a masterpiece of insight like that 500yr creation 'The Old Course' is. But I sincerely believe that golf is far more versatile an art than to be so narrowly typecast. I certainly hope some open-minded GCA types will not let traditional ideas keep them for creating on that unique canvas.
I repeat, I have never denigrated the wonderful courses that classic GCA thought has produced. I simply do not believe they are anywhere near the limit of the golf art.
Doug