A basic answer is that the weather patterns that cause tornadoes exist over various parts of the US at different times. Tornadoes are formed from supercell thunderstorms, which tend to form in the presence of wind shear--where there are temperature/wind direction differences in different layers of the atmosphere.

The geography of North America lends itself well to shear, as the typical weather pattern has masses of cool, dry air from Canada clashing with warm moist air, leading to an ideal shear environment, especially in the spring. As spring progresses, the theater for this battle between the air masses moves north and westward.

Most storm chasers, and thus most of the footage, are in the Great Plains around Texas, Oklahoma, Kansas, and Nebraska because it is a much safer environment to study tornadoes there. The terrain is flat, there are few trees, much of the area is rural, and the roads are for the most part laid out in a grid system that allows them to manuever and adjust their position much easier.

East of the Mississippi, the terrain is rolling to hilly with a larger amount of forestation. There are more towns and cities, thus more traffic, and roads have little to no order to them, often following the terrain. At best, it makes catching good footage and data harder, at worst, it's a good way to get into a very dangerous position.