incrediBILL - 9:03 pm on May 2, 2009 (gmt 0)

The reasons are way too numerous to easily fathom.

In the early days of AdSense there were fewer publishers, I now compete with many more than I used to for those AdSense clicks. Fewer publishers meant more money.

Also back in the day Google didn't have their local search they way it is now so your site showed up by default worldwide and you got more clicks, then the local defaults kicked in, gone.

Advertisers realizing they had been garbitraged pulled out of the content network.

Google killed garbitrage, profits dipped further.

AdSense targeting changes over time may or may not have altered which ads your site displayed compared to earlier years.

Thanks to Google Cache, Google Maps and other services the customer stays on Google longer, possibly sees the same ads on Google first, and perhpaps doesn't visit your site in the first place, can you say NOARCHIVE?

Blah blah etc.

[edited by: incrediBILL at 9:04 pm (utc) on May 2, 2009]