I suppose it's because when I first started buying gaming figurines, they tended to cost less than a buck apiece, and figures designed especially for wargaming were often as low as 20 cents. That makes modern metal figures seem very, very expensive to me, and when it comes to companies like Games Workshop, ludicrously expensive.
Reaper Miniatures have a very extensive range, mostly of pretty good quality, and for a 21st century company, their prices are fairly reasonable. You're still looking at six to ten yankee dollars for a 25-28mm metal figure though, which is a lot when you want to buy a whole horde of orcs or something.
They've recently started producing much cheaper plastic versions of some of their range. The plastic figure range is called Bones, and I decided to try them out. I bought 3 of the Great Worm (77006), Rats (77016 — you get 6 in a pack), and 5 packs of Kobolds (77010 — also 6 to a pack).
The detail appears a bit softer than I'd expect from metal figures, but to what extent that's due to the modelling, and how much to the nature of the medium I don't know. The detail on the Great Worm (below) looks a lot sharper, but then it's also a much larger figure.
The Rats are by Sandra Garrity, and are pretty good; the proportions and the three poses provided are suitably ratty.
The Great Worm is decently monstrous, and it's with really large figures like this that you stand to save a LOT of money. It costs $US2.99 in plastic, and I doubt that you'd get much change from three or four times that amount if it were made in metal — plus it would weigh a ton, and that means paying extra freight as well.
Of the Bones figures I have thus far, I've only painted the Great Worm, shown here with an old WotC metal figure (circa 2001, I think) for scale. The Worm does stand on its own moulded base, but it's slightly out of balance, and I glued it to a honking great steel washer for the sake of stability.
I'm pretty happy with what I've seen of the Bones range so far. I don't think metal figures are going to disappear overnight, but I do think they're going to be reduced to more of a niche market within the next few years, if only because of their expense. A cheaper alternative like these allows me to buy more figures, which means I can throw larger hordes of mooks at my players, and that can't be a bad thing.