Why did early versions of string theory fail as explanations of the strong force?

Early versions of string theory failed as an explanation for the strong force primarily because the theory's predictions deviated from experimental measurements made by the mid-1970s.1

Specifically, the theory predicted the existence of a particular massless particle that had never been observed in experiments studying the strong force. Furthermore, as researchers gained a deeper understanding of particle interactions, they developed quantum chromodynamics, which successfully described the strong force through the concept of asymptotic freedom—a phenomenon where quarks interact more weakly at smaller distances.123

Because string theory could not reconcile these experimental findings, most physicists concluded it was irrelevant to the physical universe at that time. It was only later that some researchers reinterpreted the previously "failed" prediction of the massless particle as a potential key to unifying general relativity and quantum mechanics, shifting the focus of string theory away from the strong force and toward a broader unified field theory.14