That's how it was decided I don't see any specific reasons but if you're doing single area OSPF you can choose any number you want , zero is not mandatory.If you use multipl areas though then all your areas will have to be contiguous with area 0 being the backbone.
OSPF requires that you divide your network up into a logical star of areas.
The reason for areas is the old computer science trick of "divide and conquer". OSPF routers in an area share all information about routers and link status. This uses RAM, and computing the Shortest Path First (Dijkstra) algorithm can use CPU. Furthermore, routers in an area share information via Link State Advertisements (LSA's), which are flooded throughout the area. (Chicken and egg problem: routers may not know the other routers in the area or how to get to them, so LSA's have to flood around to make sure every router gets a copy). By dividing the network into areas, the amounts of RAM, CPU, and flooding traffic are limited, providing better scalability.
The Designers thus thought of making the Backbone area as Area 0 which is just logical