Perhaps because CA has the largest economy in the united states.

I don't think anyone would care if the entire state of Texas was blown up.... >_>

I kid, I kid, (runs away from the angry rednecks mob!)

I don't think they assume,it's just that there's alot of obviously important things in those 2 places,but if there's a target that's just important,I really don't think it's going to be less protected,no matter were it's located.

Well, if they had an attack that happened on 9/11, DC and New York would be the ideal places because all the eyes would be on those places with the ceremonies. They have all these memorials and ceremonies happening on that date that it would be a big spectacle, which is probably what they want.

Well the idea is that terrorrists will most likely attack a place of high interest to the US as a whole.
-NY is a center of commerce here in the US, as well as one of the highest concentrations of populace. It is an ideal target for you can kill/hurt the greatest amount of ppl in a smaller space.
-DC, it is our nation's capital after all. Why not try to take out the pres while he is chilling in his home?
-Hampton Roads/Norfolk Virginia: This is the US's largest Naval base on the east coast(and period if im not mistaken), Think Pearl Harbor.
*AND all most of these places have been attacked before. One might think that "well if they tried that area already why would they try again knowing that security will be upped?". Tru but no one every really expects lightning to strike twice in the same spot now do they?.........

What RolStoppable said.

If it were France it would be the Eiffel Tower.
London it would be Big Ben.
Japan.....well its such a high concentration of life seeing as they are so small you can hit them anywhere and it will hurt.