It comes down to whether the stuff is perceived as lots of little indistinguishable bits, or a few big chunks. The former category (mass or non-count nouns) includes water, grain (in the sense of a quantity of grain, not lots of kinds of grains), sand, snow, etc. They are not inflected for the plural and do not take numbers - we say "three grains of sand" not "three sands". It's plausible for Lego to work the same way - so "ten pieces of lego", not "ten legos".

Perhaps in these oft-cited days of lego sets that are more like a few big pieces than lots of little pieces, people will start to think differently.