The short answer is because Constantine didn't flip a lever and make everything come out Christian.

The medium answer is that despite the Milvian bridge epsiode, Constantine's embrace of Chrisitianity both on a personal level and a political incorporation into the accoutremants of Roman imperial power was a process that continued throughout his life. Constantine adopted traditional Roman iconography and imperial trappings and only slowly added and altered these with Christian elements. His embrace of Christianity was similarly progrssive.

In this perspective, and given that the Arch was completed in a very brief time, by pagan sculptors, with the backing of the Roman Senate, I think that it is hardly surprising it lacks overt Christian symbolism.

Edit: fixed some typos.