Non-Circulating Commemoratives?

Why Indian base metal coins are under NCC? Shouldn't only silver and gold coins be NCC?

The majority are made from precious metal composition (84%) but composition is not a requirement.  A good example:  N#83828

Then what determines a NCC coin?

Definition does.  Coins issued by a government with collection or investment purpose, including non-circulating legal tender coins (NCLT). They were not intended for circulation. The coins are denominated in a currency which has some circulating coins or banknotes.

I do take issue with some of the so-called NCCs.  See my post here:  https://en.numista.com/forum/topic146084.html