Because it is in partitive. :)
"Tämä kana" doesn't refer to a (one live) chicken, but to chicken as a mass noun, and so the adjective is in partitive because we are talking about an unclear amount of chicken.
If you said: "Tämä kana on musta" (this chicken is black) it would either refer to a living chicken with black feathers, or to a whole cooked chicken that's charred.
You can spot the partitive as an added -a or -ä at the end of the word, depending on which one vowel harmony neccessitates. Front vowels only go with front vowels and neutral vowels, back vowels with back vowels and neutral vowels. The partitive ending in words with just neutral vowels is -ä.
Front vowels: ä, ö, y
Neutral vowels: i, e
Back vowels: a, o, u
jäätelöä, pirtelöä, mämmiä
ketsuppia, kanaa, pullaa
"If you said: "Tämä kana on musta" (this chicken is black) it would either refer to a living chicken with black feathers, or to a whole cooked chicken that's charred."
Thanks, that clarifies it a bit.
So if I have a whole black chicken on my plate (alive or dead ;)) I'll call it "musta" and otherwise I'll see it as chicken meat and stick to the sentence in the tips section ("if an adjective appears alone as a predicative, it is in the partitive singular whenever the subject it refers to is an uncountable noun.") and call it "mustaa".
Well, so that seems to be the - small but important - difference between "Tämä koira on musta" and "Tämä koira on mustaa" (yuck!) ;).
As you correctly point out, "kana" is the subject, and thus is not in partitive. But "mustaa", which is a description of the chicken, is in partitive because - if I've understood correctly - it describes an undetermined amount of chicken (meat).
So my question for the native speakers is: if one's talking about one individual (living) chicken, or even one specific breed of chicken, we could say "Miksi tämä kana on musta?", right?
There are different situations with the adjectives. Most of the time they follow the noun. So, if they are part of the subject, they are in nominative (no case ending - i.e. "extra a". If they are part of the object, and they qualify an uncountable noun, they will be in partitive, like the noun (and therefore get that extra a). But things are more complicated with the verb "to be", as the adjective might be a part of the subject or not. In "this red juice is sweet", "red" is part of the subject, but "sweet" is not (I believe it is called a predicative). So in Finnish red would be in nominative, but sweet would be in partitive. The same applies to sentences involving "to have", but that's because in Finnish (as in other languages - Russian, for example) is expressed in way that is close to "to be by". That's why "I have a car" becomes "Minulla on auto", using the 3rd person of "to be" ("a car is by me"). So, sen