It has everything to do with pk and the restrictions around being good.

In general the better player killers are those people who are really into that aspec of the game. By and large people who are into PK prefer to have a wider range of people they can go after. Being goodie doesn't just eliminate other goodies from the mix but often a whole slew of neutrals, and when neutrals are a big portion of the pk range you end up with only 1/3rd the oppertunities to participate in pk as opposed to a neutral or evil char (given that all aligns are equal in number aren't)

Additionally players who are more focused into the pk aspect of the game (which in all liklihood represent the largest portion of the playerbase, keep in mind focused does not always mean good at) are also likly to avoid the excess interactions expected of new age goodies. Goodies simply require more non pk oriented role play then the other aligns and that drives away people heavily interested in pk.

while I know this post is titled it has nothing to do with gear. I know personally one of the biggest drawbacks I see to playing a goodie is how the playerbase seems to expect goodies to interact with neutral mobs. Only gearing off evil mobs and via request is just not very fun or effective as a goodie.