Why isn't the word "Which" one of the 5 Wh-question words?

Which means to

used in questions and structures in which there is a fixed or limited set of answers or possibilities) what one or ones

Hence, it would be a supplement to the other ones. If John bought a cat in Springfield, you could ask which John, which cat, and which Springfield.

Wikipedia is not an authoritative source on grammar, as John Lawler has pointed out. That said, it can be useful to point in the general direction for research.

Also, as Hot Licks mentioned, Kipling seems to be in the equation.

I keep six honest serving-men,
(They taught me all I knew);
Their names are What and Why and When
And How and Where and Who.

However, the origin of the 5(6) Ws goes back to Latin "7 Qs"…in which the English Wh word is usually a Latin Qu…

Quis = Who

Quid = What

Cur = Why

Ubi = Where

Quando = When

Quem ad modem = How

Quibus ad miniculis = With what...

“Which” in Latin is Quod, and that does not appear in the original list.

The original 7 Qs were important in the discussion of ethics. Thomas Aquinas attributed them to Aristotle. More recently, they have been considered one of the basic tenets of effective journalism.

So, while Kipling may appear to be the origin of the selection, it is much older than that, and was probably included as part of the English public school (AmE private school) education. Kipling’s own education seems to have been a bit spotty; he probably picked up the idea from his mates at the newspapers he started his career with.