I'm not quite sure what you are asking, if there is one or why it is not more important - there is a unit known as the "Einstein", namely, one mole of photons. It was sometimes used to describe photon flux (einsteins per square meter per second, say) in the latter half of the 20th century. However, it is not part of the SI system as the same information can be conveyed by saying "one mole of photons" instead of "one einstein".

The reason for why it is not widely used unlike other technically redundant units, such as the electron volt or atomic mass unit, is that there is nothing special about a mole of photons; it doesn't correspond to any common process or scale or anything like that. An electron volt (the energy needed to move an electron across a potential difference of one volt), on the other hand, is useful because an electron moving across a potential difference of one volt is an intuitive and not untypical process at certain scales. Similarly, an atomic mass unit is roughly the weight of a nucleon.

If there was a particularly intuitive unnamed quantity to express phenomena within relativity or the photoelectric effect that couldn't be easily given in terms of existing units, there's a pretty good chance it would indeed be known as "the einstein", much like how certain quantities in quantum mechanics are often given in terms of multiples of Planck's constant, say.