It's unnecessary, piston aircraft made way for turboprops in the 60's so a splitting fuel makes very little sense for the remaining years in the game. So Avgas would become redundant. I can't name a piston aircraft in the game off the top of my head after the 50's/60's.
If you were to add both fuels, the logical step to make at the same time would be to model which airports have which fuel and the fact that Avgas is much harder to find these days compared to the past.
The other reason apart from simplicity is simply oil prices, both are made from the same raw product, I would therefore expect prices to not be that different and any rises are the same for each, broadly speaking. So it doesn't make sense to complicate the game this way.