The drop from 2003 on probably has a bit to do with disposable income..
Check the unemployment rates for the same time
now Interest Rates
Now in the last few years unemployment rates have dropped but interest rates are on the rise.
The mortgage market is a mess and home values are sinking.
So people don't have the extra cash to keep flying.
Looking closer I see something that I can't explain.. when unemployment is up so is membership..