Lightspeed is different depending on the material it is passing though. That's some of the motivation of ether theorists too, to use ether as a material with optical properties to explain why the speed of light is 300 million m/s and not, say 450 million m/s.

That's a question I have had for a long time: Not just - why is the speed of light constant - but - "Why is the speed of light 300 million m/s and not some other value?" What was the speed of light in the early universe, before it became transparent to light? What is the speed of light in the core of the Sun? What about a universe filled with predominantly empty space and supercold bose-einstein condensates?