Remember that becoming an ace requires downing five enemy aircraft, and to down enemy aircraft the enemy needs to have some in the first place.

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The VPAF had only started to receive jet fighters in 1964, and their aircraft were generally less numerous and less capable than what the Americans were fielding. Because the aims of the VPAF were exclusively defensive in nature and their supply of both manpower and materiel was very limited, they would deploy their aircraft very conservatively. They generally would only sortie their fighters when the situation was very much in their favor, and tactics were designed around ambushing strike formations, with forcing the attackers to drop their ordnance early and abort the mission being just as effective as downing an American aircraft. A more typical tactic involved having fighters appear away from a strike package to draw away escorts and then having another group pop in under the radar and make a pass on the strike aircraft themselves. While the typical strike aircraft (F-105) was nominally faster than the MiG-17, it could only outrun the MiG-17 once it had jettisoned its payload and thus ruined the mission. The MiGs, on the other hand, didn't exactly stay around to fight - all they had to do was ruin the mission, so a typical "attack" could consist of one pass on the strike aircraft followed by a rapid withdrawal by both the attacking and decoy aircraft.

Because of that, opportunities for American fighters to actually engage the VPAF were few and far between. Making this worse, particularly early on, was the tendency of American forces to rotate pilots through the theater to spread experience and the generally poor air-to-air combat training provided early in the war. So while encountering an enemy fighter was already a rare thing for American pilots in Vietnam, the rotation of aircrews meant that it would be rare to stay in the theater long enough to encounter enough VPAF fighters to even have the potential to make ace, and poor air-to-air combat training meant that you were less likely to be successful at downing an enemy fighter before it was able to disengage and escape.