Have mentioned this here and there in various publications, but the reason are basically only medium-weight .25 (and .270) bullets is because EARLY smokeless cartridges used very long, round-nosed bullets, apparently because the black powder bullets they replaced used very long, round-nosed bullets. The bullet diameters developed back in the 1890's mostly were military rounds, especially 6.5mm, 7mm and .30, which all started out with such long round-nosed bullets, such as the 156's in the 6.5x55 and 220's in the .30-40 Krag. However, some early sporting cartridges also featured such bullets, especially the .33 caliber British rounds. All of which is why 6.5mm, 7mm, .30 and .33 caliber barrels had relatively fast twists from the beginning.
By the end of the 1890's, however, lighter spitzers started appearing, the reason the original 220-grain round-nose for the .30-03 became the 150-grain spitzer for the .30-06. Lighter spitzers and higher muzzle velocities became the trend in sporting cartridges, often with slower twists, just fast enough to stabilize the bullets, apparently because the bullets of the day weren't very well balanced, and a faster twist made them shoot noticeably less accurately.
The .250 Savage originally had a 1-14 twist for 87-grain bullets at 3000 fps, the .270's 1-10 twist was developed for 130-grain bullets, and smokeless .35 caliber cartridges has 1-16 twists, just enough to stabilize 250-grain spitzers. This is why 6.5mm, 7mm, .30 and .33 caliber barrels all have had twists sufficient to stabilize relatively heavy spitzers, but some calibers (especially .25, .270 and .35) have slower twists.
Savage didn't change the 1-14 twist in their barrels to 1-10 (the standard twist in the .257 Roberts and many other .25's) around 1960, though some other companies kept the twist 1-14 long afterward. The original .257 Weatherbys had 1-12 twists. Both 1-12 and 1-14 won't reliably stabilize any 115-120 grain spitzers, the reason the "heavy bullet" factory loads for both rounds, used 117 round-noses--and until very recently Weatherby still offered a 117 RN factory load.
One oddity about all this is the original twist in the .25-35 Winchester was 1-8, which is what my .25-35 Model 1894 rifle (made in 1898) has in its octagon barrel. If newer .25 caliber cartridges such as the .250-3000 or .257 Roberts has stuck with 1-8, then .25's might well be just as popular as 6.5's for "long range hunting."
