Admittedly I wouldn't be the best person to ask as I haven't studied science in nearly a decade, but my uneducated educated guess would be a change in the value of c is balanced out by other factors, probably something to do with dark energy.

In a sense the changing rate of expansion of the universe due to dark energy is what is causing the change in c. But that's just 100% made up of the top of the bonce. Hopefully someone with a more advanced and recent education can give you a good answer.

Key point is things don't change just because. There's always a reason fundamentally