The switch from 6.5 was brought on by reports coming from Japanese infantry that when they shot someone they just kept coming (sound familiar). I believe part of the actual quote mentioned Hollywood. Apparently they were expecting an approaching soldier to fly backwards and bounce off a bar room wall after being shot, but it didn't happen. I believe the original bullet was 160gr round nose projectile and they later switched to the 139gr bullet to get greater velocity and effectiveness at shooting through heavy bamboo brush in the pacific theater. I might be confusing the 6.5J with the 6.5 Carcano on the 160gr issue, I'm too tire to look it up.
Most of the European and Scandy countries adopted the 7.62 simply for NATO, even some of the countries that didn't participate in NATO. The 6.5x55 Swede is a fantastic hunting round used all around the world, though most of the loads are at far less than capacity, simply because the older weapons the round was designed for cannot handle higher pressures.