A few people here have touched on the real answer to the title question. It was a combination of plants/entryists (some of them exerting extraordinary influence on Gormley & co), ex-PDs,
PD-minded chancers, and a few FFers probably, all of whom joined after the Greens took six seats in the 2002 general election. Why didn't the Greens go into coalition in 2002? They could have if they really wanted to. But they still had an attachment to their principles and policies then.
After 2002, the Greens were so swept up in their sense of power that they were careless about who joined. And the wrong people joined, and started throwing their weight about in the party. Gormley & Sargent were so spineless and so eager to get into government that they found it easy to cave in to those vocal unGreen newbies, and everything could be excused by the need to "save the world". Once they started lying and dissembling they couldn't stop. Green policies (carefully developed over many years) became irrelevant, or were abruptly deleted/changed to suit
FF. Soon it became easy to ignore the core principles too...
Now it has become a case of "four legs good, two legs better".
How to destroy a party in one easy lesson.