Light travels about 186,000 miles/sec. in a vacuum. Why this speed, and not some other speed?
This is a great, simple, question that touches the heart of modern physics.
The simple answer is that we don’t know. The speed of light, c, is one of the “constants of the Universe” that we can measure, that by assumption and experiment are constant everywhere and at all times in the observable Universe, and that we have no means of deriving from deeper principles. Other quantities of this ilk are the Newton/Einstein gravitational constant, G, Planck’s constant, h, that determines the scale of quantum uncertainty at the microscopic level, and the masses of the electron and a small host of other particles. These quantities are measured to the best of our ability and plugged into the Standard Model of particle physics and other contexts throughout science and engineering. There are ways to test whether c, G, and h, are the same far across the Universe, and they are to within experimental accuracy.
Having said that, physicists ache to determine those “constants” in some more fundamental way. There are issues of why these constants are “tuned” the way they seem to be to allow life in our Universe. Efforts to unite Einstein’s theory of gravity with the quantum theory have brought string theory with its demand for multiple spatial dimensions and suggestions that there could be a “multiverse” in which each component had different values of the “constants of the Universe.” There is plenty of thinking going on but no firm guidance yet.
For more on the topic, you might look at the provocative book “The Life of the Cosmos,” by Lee Smolin or “The Cosmic Landscape” by Len Susskind, both accomplished particle physicists and great writers. Wikipedia can also be a guide:
Prof. J. Craig Wheeler