Why Bosonic String Theory Doesn’t Describe Our Universe
The early string theorists had therefore spent a lot of time giving meaning to a theory that seemed to (almost) accurately predict the Smatrix, only to find that the majority of particle physicists weren’t interested in it. It had to be very frustrating to have such an elegant model that was quickly falling into obscurity.
The dual resonance model was created for the express purpose of explaining the Smatrix particle scattering, which was now explained in terms of the Standard Model of particle physics — gauge fields and quantum chromodynamics. There was no point to string theory in light of the success of the Standard Model.
Also, as the measurements of experiments in particle accelerators got more precise, it became clear that dual resonance models were only approximately correct. In 1969, physicists showed that Veneziano had discovered only the first term in an infinite series of terms. Although this term was the most important, it still wasn’t complete. The theory appeared to need some further refinement to match the results perfectly.
Terms could be added (which Michio Kaku did in 1972), correcting for the different ways that the strings could collide, but it made the theory less elegant. There were growing indications that string theory might not work the way everyone had thought it would and that, indeed, quantum chromodynamics explained the behavior of the particle collisions better.
But a few string theorists weren’t about to give up on it quite yet. By 1974, bosonic string theory was quickly becoming a mathematical mess, and attempts to make the theory mathematically consistent caused more trouble for the model than it had already. Playing with the math introduced four conditions that should have, by all rights, spelled the end of the early string theory:

Massless particles

Tachyons, which move faster than the speed of light

Fermions, such as electrons, can’t exist

25 spatial dimensions
The cause of these problems was a reasonable constraint built into string theory. No matter what else string theory did, it needed to be consistent with existing physics — namely special relativity and quantum theory.
The Standard Model of particle physics was consistent with both theories (though it still had trouble reconciling with general relativity), so string theory also had to be consistent with both. If it violated a half century of established physics, there was no way it could be a viable theory.
Physicists eventually found ways to modify the theory to be consistent with these existing physical laws. Unfortunately, these modifications resulted in the four problematic features outlined in the bulleted list. It wasn’t just that these features were possible, but that they were now seemingly essential components of the theory.