Well, there was the initial reason that it suited my business model better than metal but resin is awesome stuff for minis.
I love the lack of shrinkage. When you cast in metal the figure shrinks and flattens slightly and can end up not really looking quite as the sculptor intended. Resin castings are as close to the original sculpt as you can realistically get. And that means that multipart figs tend to fit together better.
Surface. I love that you have the lovely smooth surface on resin rather than the often rough quality on metal, particularly on larger pieces (my 54mm Dracula would have been nowhere near as nice in metal).
Weight. It weighs very little. Now I know some people like the weighty feel of metal but that can be aproximated by adding a coin or similar under the base. The upsides to the lack of weight are that I can ship less expensively and, from a modelling perspective, multipart figs require less pinning.
Ahhh resin. I genuinely believe it's the best choice for single miniatures as I sell...
I'll give metal the benefit of the doubt for producing units and armies though it's got a bit too expensive to be a bargain...
... and the setup costs for plastic are enormous.
Zombies
Approx 31mm foot to top of head
Three unpainted metal figures supplied with plastic bases