Bell's preferred calibre was the 6.5x54MS. He stoped using it when he ran out of reliable ammo and had to switch to the 7x57. This is in his book.

The adoration of the 7x57 is due to his chance use of it and supply difficulties of 6.5x54. Not through choice. He started off with .303's as he had no money.

It is a lot cheaper to buy and carry more ex-mil ammo than find and buy large bore ammo. Bell was not recoil shy. This also is in his writings. I believe this supply issue may be the reason for small calibre rifles being used.

He had many and varied travels and employments during his life. Including gold prospecting and supplying gold diggers in Alaska (?) ref needed.