Why? As it still is.

Distances in the universe are measured by the time light needs to travel them. If a photon arrives at us from the "outer rim" of the universe (the outmost part of the universe that is yet able to interact with us), the time it needed for that is much longer than for the original distance it would have had to travel once it started. Which means that the distance it had to travel increased. That is what we call the expansion of the universe.

The thing is, the speed of light is no limit for the expansion of the universe, just for everything in it. The expansion may seperate objects faster than the speed of light in which case light from one can no longer reach the other. While a photon was crossing a meter trying to get from one to the other, the expansion added more than a meter to the remaining distance.

The other thing is: that the photon needs more time to reach us doesn't mean it swings more often along its voyage. So its frequency gets lowered/its wavelength increased (as the product of frequency and wavelength remains constant being c). It is redshifted. As the additional distance it had to travel is proportional to the original distance the redshift becomes stronger the further away the voyage started.