Roughly because of Lorentz boost, aka length contraction. So if I take some distance, say 1m, and I look at it from relativistic speed it gets contracted. And if I do that from high enough speeds, it gets contracted below the Planck length, so a fundamental length constant would violate Lorentz symmetry.<p>Some people tried to construct such a theory, but as far as I understand, they are going nowhere. See<p><a href="https:&#x2F;&#x2F;en.wikipedia.org&#x2F;wiki&#x2F;Doubly_special_relativity" rel="nofollow">https:&#x2F;&#x2F;en.wikipedia.org&#x2F;wiki&#x2F;Doubly_special_relativity</a>