Most important part-possibly, but at different times you could also make a case for a couple of the caliphates, the Mongols, Vikings and Normans. It would probably be better to say the more enduring major role, since the Eastern Empire was a major player from Constantine's time until the Fourth Crusade, and then managed to limp on for over two centuries more before falling to the Ottomans.

Problem is that important or enduring does not entitle a subject to production. It comes down to manufacturers make what they want or what they think will sell. Unfortunately the Eastern Empire does not attract as much attention as the classical Rome or WW2. When was the last time you saw a bad Hollywoood movie based on the Byzantine empire?

How many Hollywood movies did you see on Seljuk Turks or VIII Franks ? Strelets did them...
Once should also remember that there are many collectors asking for this subjects for years. In 28 mm scale Byzantines are very popular –look at Cruder Sets or Old Glory. Besides Orion Byzantine set in spite of its poor quality was also very welcome ….

Well that should have taught me not to answer the question asked, but I guess I am a slow learner.

I cannot recall anything on the Seljuks, but their successors were major players in the Crusades and there have been plenty of movies/shows on that. For the Franks only Strelets knows for sure.

We all count on Russian Manufacturers for Eastern European subjects. I'm sure we'll not be disappointed, sooner or later.