The short answer: the US Army.
Essentially, the US Army felt that the M-1 Garand had been a very successful rifle, and wanted to continue using a full-power rifle cartridge. Also, keeping such a cartridge would allow them to use the same ammunition in their rifles and machine guns.
Other Western countries were in the process of switching to intermediate cartridges and true assault rifles. The British had accepted and started issuing the British-designed EM-2 bullpup rifle, which fired a .280 (7x43mm) cartridge. The ancestor of the FN FAL was developed as an intermediate-cartridge assault rifle.
However, US obstinacy and desire to keep a full-power rifle cartridge combined with the evident importance of standardising NATO rifle ammunition (and ideally the rifles, too) led to NATO adoption of battle rifles as their standard rifles.
For more on this story, see:
u/Brezhn3n in https://www.reddit.com/r/AskHistorians/comments/7ghw2m/why_did_the_us_not_begin_to_develop_their_first/
Once the deficiencies of automatic fire with full-power battle rifles were made clear in action, NATO moved to intermediate-cartridge assault rifles.