Final Fantasy XIV Database - Ffxivpro.com

In my opinion and to put a long story short, I believe it's a mixture of:

RMT's gradual decline from the inflation days

A major lack of patience and general understanding of economic basics among the entire FFXI community, thus undercutting, etc.

SE pulling the plug on numerous legitimate gil earning methods

The gap between low, mid and high level activities as well as the difficulty and lack of assistance to aid more people into obtaining things such as sky, sea, etc.