As for your question about demand vs. inherently higher BC, the answer is both.
Certain calibers have always included some very high BC bullets, not because of anything inherent to the caliber but because certain bore diameters were matched with very heavy bullets during the early days of centerfire, smokeless cartridge development. The best examples are 6.5 and 7mm.
Several old 6.5mm and the most popular 7mm military rounds used very heavy-for-caliber roundnosed bullets. Examples are the 6.5x55 with a 156-grain and the 7x57 with a 173-grain. Because of the long bullets, rifles for both rounds were given twists of 1-8 to 1-9.
Consequently, when spitzers came along the 6.5x55 and 7x57 could stabilize very high-BC bullets, thanks to their faster rates of twist over other early centerfire rounds such as the .257 Roberts and .270 Winchester, which used lighter-for-caliber bullets that stabilized in 1-10 twists. After that all 6.5 and 7mm rifles received faster twists than most other calibers, the reason both became popular as long-range calibers.
Fast-forward to the present, and we find the demand for high-BC bullest extending to all calibers--including some bullets that are too long to stabilize in the standard twists of some calibers. This also created a demand for faster-twist barrels, the reason we now see a lot of .224's and 6mm's with 1-9 or faster twists, and even some .25's and .270's.
