Some small companies ONLY put out metals. Lots of companies have lines in various materials. For example, Reaper has their metal Dark Heaven, Pathfinder, etc, but ALSO puts out PVC models in their bones lines. So if it bothers you if they put out ANYTHING in another material, your options are more limited. Most of the ones listed above in the previous post have put out something in an alternate material, even if most of what they put out is metal.
Metal declined in use in part because China controls the tin market, or at least did for a while. They made the price fluctuate, so it made pricing models harder when you didnt know what you were going to pay for
raw materials 6 months later.
In general, larger companies use whatever material suits the model best. Some sculpts simply aren't feasible in metal due to the weight. I noticed you saw Darklands was converting some of their line to metal. However they really cant convert all of it. Krull is a huge model and costs almost $200 in resin, and is I think about 6-7 inches tall, with pretty massive wings
http://mierce-miniatures.com/index.php?act=pro&pre=mrm_dkl_inf_dis_wld_101_000
He'd probably weigh about 6 pounds in metal, and the cost would go up even more. Resin seems more expensive for smaller, man sized units because you have to babysit the casting process. So more labor intensive, but less costly in materials. As the model size increases, the price of materials exceeds the additional labor cost, so large metal gets expensive fast. I would guess Krull would cost $300 or more in metal.
However you also have sculpts which aren't feasible in metal. Wyrd for example, has lots of smaller parts. With plastic cement, you can weld them together. However the bits are too small to pin, so a metal version of the same sculpt would be very fragile.