Why Is Everything Always Set in New York or L.A.?

The Chicago River at night, perfect for TV

Chicago is the "Second City," but on television, it is definitely behind New York and Los Angeles.  But I wanted to know just how far behind, so I looked up the current television lineup for all the network shows and determined their settings (I did not count reality shows, news programs, or game shows).  Many shows locations are unknown, such as The Simpsons, but others do have a setting.  What I found out was somewhat startling:  Chicago tied for dead last in the number of shows set at a location.  Here is the location list from most to least:

1. Los Angeles/California: 11 [90210, Brothers and Sisters, FlashForward, Melrose Place, NCIS: Los Angeles, Numb3rs, Private Practice, The Big Bang Theory, Two and a Half Men, The Mentalist, Trauma]
2. New York/New Jersery: 9 [Mercy, 30 Rock, Castle, CSI: NY, Gossip Girl, How I Met Your Mother, Law & Order, Law & Order: SVU, House]
3. Virginia/D.C. : 4 [Bones, NCIS, The Cleveland Show, The Vampire Diaries]
4. Miami/Florida: 3 [Cougar Town, CSI: Miami, Miami Medical]
5. Pennsylvania: 3 [Cold Case, Three Rivers, The Office]
6. Indiana: 2 [Parks and Recreation, The Middle]
7. Chicago: 1 [The Good Wife]
8. The Rest Where a Location is Known: 11

To me, this was very surprising as Chicago is the third largest city in the United States as well as having previously had a fair number of shows set there (Perfect Strangers, Family Matters, Chicago Hope, ER, etc).  What was perhaps the most startling to me was that Indiana had two shows set there.  Granted they are both in fictional towns, but come on, the Hoosier state rarely gets any TV love so I'm counting this.