Because it was only tested on females aged 9 through 26. The FDA didn't approve it for older women because it hasn't been tested on older women. This seems rather conservative to me, but it makes sense. You don't approve a drug for a patient that it hasn't been tested on.
Why did Merck choose 26 as the cutoff age for their study? I don't know. Maybe they did some complex statistical stimulations of exposure to HPV, or maybe they just drew the number out of a hat. But it's the number they went with, so it's the upper limit we're stuck with -- until and unless more tests are done on older women.
Safety reasons aside, it is better to administer the vaccine to younger females just to cut down on the adverse reactions. In some cases, Gardasil was reported to cause painful periods. Administering it to prepubescents pretty much nullifies that risk. It also provides a convenient dodge for any parents who don't want to tell their kids about all the sex they can have in high school. ("This shot protects you against a certain kind of cancer. You can't get this cancer until you're much older, but the shot can mess up your periods, so we're giving it to you now.")
I wish I would have known more about STDs when I was younger.
My parents really didn't tell me anything about sex. I did not know anything about STDs until I was in my 20s and all of the aids stuff came out.
I've already started educating my 12 year old son about STDs. He already knows a lot more about sex than I did until I was much older.
I have 10 year old daughters, and I think it's great that there is a vaccine for HPV. I just don't know that I am willing to take the risks for it right now. When they are 16, and the vaccine has been around for 6 years without major side effects then I may feel differently.