Maxwell made a rigorous, mathematical study of the properties of electricity and magnetism, and he proved that there must be a phenomenon that he called electromagnetic waves. According to Maxwell's theory, an electromagnetic wave must propagate at a constant speed that he called $c$, and which could be calculated from other physical constants that were known and measurable at the time.

Around that time, scientists were actively debating the nature of light. After Hertz demonstrated the existence of electromagnetic waves in a laboratory experiment, Maxwell's theory suddenly became the most favored explanation for light.

The funny thing about Maxwell's theory was, that the speed was relative to whoever measured it. If you and I both measured the speed of waves emanating from the same source, we should both get the same result, regardless of our motion relative to the source or, to each other.

Some physicists found that to be troubling, and they tried different ways to justify it. Einstein's big achievement was to combine their various ideas into a single consistent, rigorous, mathematical theory. One of the consequences of his theory---proven in the math---is that if anything has a characteristic speed that must be the same for every observer, then nobody can ever observe anything moving faster than that speed.

So, the universal speed limit is a mathematical consequence of certain measurable, and as far as anybody knows, fundamental physical constants; (e.g., the permeability of free space https://en.wikipedia.org/wiki/Vacuum_permeability)

Your question then comes down to, "why does the universe have those properties?"

Every time physicists answer a "why" question, the answer always rests on deeper levels of "why?"

Maxwell made a rigorous, mathematical study of the properties of electricity and magnetism, and he proved that there must be a phenomenon that he called electromagnetic waves. According to Maxwell's theory, an electromagnetic wave must propagate at a constant speed that he called $c$, and which could be calculated from other physical constants that were known and measurable at the time.

Around that time, scientists were actively debating the nature of light. After Hertz demonstrated the existence of electromagnetic waves in a laboratory experiment, Maxwell's theory suddenly became the most favored explanation for light.

The funny thing about Maxwell's theory was, that the speed was relative to whoever measured it. If you and I both measured the speed of waves emanating from the same source, we should both get the same result, regardless of our motion relative to the source or, to each other.

Some physicists found that to be troubling, and they tried different ways to justify it. Einstein's big achievement was to combine their various ideas into a single consistent, rigorous, mathematical theory. One of the consequences of his theory---proven in the math---is that if anything has a characteristic speed that must be the same for every observer, then nobody can ever observe anything moving faster than that speed.

So, the universal speed limit is a mathematical consequence of certain measurable, and as far as anybody knows, fundamental physical constants; e.g., the permeability of free space.

Your question then comes down to, "why does the universe have those properties?"

Every time physicists answer a "why" question, the answer always rests on deeper levels of "why?"