Why were us bombing raids in Vietnam ineffective?

The US feared that Soviet and Red Chinese technicians might be at certain "sensitive" areas, such as Surface to Air Missile radar installation sites and re-supply depots, as well as Soviet/foreign merchant ships in North Vietnam's harbors. So for a period of time those areas were "off limits" for bombing. In addition, this strategy allowed the Communist gunners on the ground to predict our flight paths. They could set up their defense systems, knowing where we wouldn't go, and have a pretty good idea in which direction we would fly in for the attack. It was similar to duck hunting; the hunters (duck shooters) were shooting the ducks as they flew over the decoys. The anti-aircraft crews were shooting at jets as they flew over the targets.

The second reason, and with all criticism aside; with every modern cannon, machingun, and missile being fired at our aircraft all at one time, jet aircraft are going to be shot down! Nearly 2,000 were lost, many with the pilots and crewmen that went with them. Those losses, combined with the restrictions, contributed to our ineffectiveness.

The gloves came off during operation Linebacker II however, in 1972. History sometimes refers to this operation as the "Christmas Bombing." We lost 15 B-52's in one week! But the bombing WAS effective that time!