For a number of reasons; The reason tank destroyers were so plentiful in the first place was the fact that they were built on the chassis of captured tanks from France, Czechoslovakia and the Soviet Union as well as older, outdated panzer chassis (an example of this is the Panzer 38(t) chassis being used to build the Marder III https://en.wikipedia.org/wiki/Marder_III).

However, while the tank destroyers were a cheaper alternative to tanks and faster version of their static cousins, they could never fulfill both the support or the offensive role a tank could in the German combined arms doctrine (this role was filled by the tank destroyers aggressive cousin, the assault gun https://en.wikipedia.org/wiki/Assault_gun) . Even looking at a picture of the Marder III, one can see it's open topped, making the crew very vulnerable and very weak side and rear armor made it ineffective in any sort of offensive role. It did however, excel in defensive, where it could make full use of it's speed and incredible main gun to full effect in the elastic defense doctrine that was endemic in German tactics in the later periods of the war. But tanks would always prove to be better options as they could fulfill more roles, especially on the offensive.