Warm climate is the largest factor for agriculture west of the 100th Meridian.
The 100th Meridian cuts right through central Texas, Oklahoma, Kansas, Nebraska and the Dakotas. Everything from this line going east usually has fluctuating weather patterns throughout the year, which isn't good for farming. The weather on the 100th Meridian and west of it (particularly south) has constant weather annually, typically around <em>50°F - 90°F</em>. It is also the border the from the colder eastern states and the hotter western states, so it's the perfect balance between excruciatingly hot, and bitterly cold.
Another large contributor to the massive agriculture out west is the fact that there are a lot of <em>wide, open plains</em>, very good to start a farm on, as wide flat surfaces are perfect locations for farmland.
Finally, if you really want to get technical, <em>soil composition</em>. A major problem with finding good farmland is finding rich soil capable of plant growth. So, in the Mojave Desert, might not be a good idea to start a wheat farm, but in the plains of Kansas, it's perfect.
Hope it helps!
