They wouldn't give up. Also, it is difficult to suppress a guerrilla campaign when you can't always identify your enemy. Also the American public got fed up at the number of body bags coming back. Don't forget, America's involvement lasted over 19 years and unless a lot more determination was shown, there seemed to be no end in sight. I used to participate in a forum which included a number of USA and USAF veterans who. virtually to a man, complained that they were having to fight with one hand tied behind their backs, and the lack of political will to 'get the job done'.
Of course the opposition wasn't just guerrillas, the NVA were a formidable opponent in their own right. Also, the ability to use Laos and Cambodia as no-go areas for the Americans (not always strictly true of course) to bring troops and war material down south free from interference helped.
Had the American military been given carte blanche, they could have reduced North Vietnam to a smoking ruin - but that wasn't politically acceptable.
Mike