Many people new to Japan are often surprised that on one hand while menus across the country look pretty similar, there certainly are styles and seasonings that vary from region to region.
Almost everyone is familiar with the "omiyage" which are supposed to represent what is "famous" from a specific area, and which are usually food and packaged in a way to be made convenient for the obligatory present to co-workers upon return from a vacation. These "famous products" from various regions usually have historical basis, but a lot of hype is required to keep them going.
Another regional variation most people are familiar with are items like ramen or okonomiyaki which differ by region - Sapporo ramen, Osaka okonomiyaki, etc.
Most food companies on the other hand are faced with more subtle variations which can make or break a product. Many products are limited to Eastern or Western Japan simply do to their appeal in one of those areas.
Major, nation-wide food companies often introduce new products in just one regional market 1) because that area was found most promising in consumer research, 2) in order to evaluate a product in a test market with specific characteristics (Shizuoka is known for being somewhere between Osaka and Tokyo in its preferences and is thus the preferred test market), or 3) with no intention of going nation-wide (usually these are seasonal and are marketed to make the people of that region feel special - people in other regions cannot buy the product).
I am not sure which of the above applies to the new mushroom burger from Mos Burger, but it is only being introduced in Western Japan. It is said to have an Autumnal flavor. Announcements like this are hardly rare, but they are interesting to follow up on. Chances are those of us in Eastern Japan will not get a chance to buy this mushroom burger even though there are Mos Burgers on almost every other corner.