Well, one reason would be that your web font would be 134Mb or so? (looking at the size of the comprehensive Noto download)

The other is simple practicality - these things take time to develop, you can either wait until all the glyphs are done, or release subsets that cover languages as you work; a subset that covers part of a language isn't very useful but subsets that cover whole languages are.

They're not really "for" a language. They are optimized subsets of the same font that only contain glyphs from one or more languages to minimize the file size.

If your website is written in English and an occasional accented character from other Western languages, there's no need to load a 50MB web font containing all the Tradntional Chinese characters.

Part of the answer is that Unicode has a lot of characters, and web pages use only a few, so for web fonts it makes sense to have the user download only ones likely to be used.

Another part is that some CJK characters look somewhat different in C, J, and K. http://www.unicode.org/faq/han_cjk.html#3

CJK unification in Unicode means that you don't know how to render a Unicode codepoint without also knowing the language of the text - the same Unicode codepoint looks different when rendered in Chinese, Japanese or Korean.

There are technical reasons for this. For example m, OpenType only supports 64k glyphs within a font. Which is enough for the BMP but nothing else (and counting ligatures that are necessary for, e.g. Arabic, it might not even suffice for the BMP).

Then there are practical considerations. While Latin, Greek and Cyrillic are similar enough to warrant the same styles (serif, sans-serif, script, italic, and various weights) not all of them make very much sense for, say, CJK or a variety of other scripts. So having different fonts for different scripts that are still designed to go together is actually not that bad a solution.

It does mean that for good typography you need a matrix of fonts based on style and script. Word includes two fonts per style for this, to treat CJK differently, which might not be enough, depending on the numbers of different scripts involved in a single document. But a) several dozen scripts per document are somewhat rare apart from Wikipedia's language list per article and font demonstrations; and b) good typography needs effort, this won't change.

I would guess that it's because of the amount of work required to design glyphs for every character that retain the font's style while still capturing the character's look and meaning from its language.

According to the site all the fonts together are 134MB compressed. Maybe that's the reason or maybe because it's a work in progress so works out better in segments.

A typographer typically works within a small set of languages. It would be unfeasible for a single type foundry to cover every possible glyph with the same consistency.

A lot of fonts don't support a lot of unicode characters.