Historically it was two factors - all that you cite, AND a change in propellant powder from stick powder (which burns relatively clean) to ball powder (which doesn't) because of budget reasons. Blame RULE FOUR REDACTED TO PROTECT THE CENSORTRON'S EARS McNamara.
Huh, side note, spoiler tags don't work if your text isn't black. Note for the future!
Where was I - oh yeah. Between the removal of chrome lining on the barrels, cleaning kits, and other adjustments to the rifle declared by SEE PREVIOUS and the higher-fouling powder switch, yeah, the rifles had issues. MOST of which, on their own, wouldn't have been too bad and overcome without too many issues in the field. All of them at once was a problem, yes.
Side note, when you do buy an AR-type rifle, or frankly any other rifle that has the option, chromium lining on the barrel (not chrome-moly barrels, that's something else) is always a good option. Much harder to get dirty, much easier to clean. Worth springing for unless you groove on cleaning solvents and brushes. (Who am I to judge?)