I go to the grocery store 1 or 2 times per week in Texas. It seems that most times I go, the fruits and veggies are from a different place. One week the grapes are from California, a few days later grapes from Chile are on display, then the next week they are from Mexico, etc.
I guess the fruit and veggie buyer has to choose on a daily basis from what is out there and finds the right combination of quality vs. cost. I am guessing that the guy selling the California stuff is putting forth a good sales pitch. One of my neighbors from a few years back was a vegetable broker who specialized in cabbage (crazy, huh?). From what he told me, he would call the suppliers (farmers, co-op reps, produce terminal operators), somehow secure a truckload or three, then would try to find a buyer, and then try to figure out how to get it there. Othertimes he would have a buyer and would have to go out and find what the buyer needed. He was a cabbage contractor.
Another obvious reason is that sometimes the local crop is not plentiful or there is not a lot of good quality stuff (usually because of not enough rain or rain at the wrong time). This happens and we might not even know it. The grocery store has got to get it from somewhere.