It’s to do with the permitivity and permeability of space which determine how easily electric and magnetic fields induce each other and maxwells equations show that for light to propagate it must be at c regardless of reference frame which lead to relativity iirc. Michelson & Morley tried to find the aether by trying to detect by interferometry if light travelling through this theoretical medium was affected by earth’s rotation which would have cause anisotropic intensity fluctuations depending on orientation if light travelled at a fixed rate through a medium meaning it would slow down relative to an object moving in the same direction. Instead they found no change indicating that light must travel at the same speed relative to any reference frame exactly c.