grammatical number - Why don't we use the indefinite article with 'software'?

Generally, one doesn't use the indefinite article with a noun because it's plural, but sometimes you get nouns where, for some reason, the indefinite article isn't used even though the noun is singular. I would never say 'a software', though it seems to be a singular noun, and I've heard it said (though it sounds wrong). Instead, we say 'some software' or 'a piece of software' (the latter implying that software is singular), so why does 'a software' sound wrong?

EDIT: I really meant to ask why we don't consider software singular. Maybe I should have asked, 'why is software a mass noun?'

Answer

That's because software is uncountable, or a mass noun.

In English, mass nouns are characterized by the fact that they cannot be directly modified by a numeral without specifying a unit of measurement, and that they cannot combine with an indefinite article (a or an).

Edit: in reply to your edit, the key here is -ware:

1. Used to form nouns denoting, collectively, items made from a particular substance. glassware


2. Used to form nouns denoting, collectively, items of a particular kind or for a particular use. giftware

Note the word "collectively". Software was coined in 1953 by contrast with hardware. You wouldn't say "I need two new hardwares", either.