This is a large retail display poster. It is big, likely about the size of a movie release poster. The glasses you see in the corners are full size (not shot or small) Posters like this are generally made out of heavier stock paper than posters for the public/general release. A poster like this would be given out by Sega before the game release (or at it) for retailers to use to decorate the store & promote the game. Remember, in the time of Sonic 1, there were many different offline games retail stores, not just huge ones or chains that would easily put up a poster like this.
This poster is uncommon. Posters, being paper, are prone to damage, easy to throw away, and may fade in the sun. Being a retail-promo item, it also would not have had the amount of production that a regular release would, contributing to the rarity. The original artist who did the artwork has died. The original piece was thought to be a painting work, about poster size.