How come the Air Force doesn't visit my school as often as the other branches?

We have military recruiters that visit my school every month. The Marines come every month, the Navy comes about every 2–3 months and the Army comes every 1–2 months. The Air Force only comes about once or twice a year and I was wondering if there is any particular reason for that. I’ve tried to ask counselors at my school and they don’t know. Is it because the Air Force is more selective? I’m really interested in the Air Force so it is slightly frustrating.