Because Afrikaans usually uses watter when English would naturally use which before a noun.
So:
Using wat directly before a noun is generally not the standard form here. A learner should think of watter as the normal word for which + noun.
In Afrikaans, question words normally go at the beginning of a direct question.
So the structure is:
That gives:
This is similar to English:
The version Dit is watter fase? would not be the normal way to ask this question.
Afrikaans follows a strong verb-second pattern in main clauses. That means the finite verb usually comes in the second slot.
In this sentence:
So:
This is a very common Afrikaans pattern and is worth getting used to early.
A simple learner-friendly approximation is:
VUH-tur FAA-suh is dit
A few notes:
If you want a rough breakdown:
Usually no, not for the same meaning.
If you say In watter fase is dit?, that sounds more like:
That version would only make sense in a context where something is in a phase, and it is less natural for the basic identification question. For the sentence you were given, Watter fase is dit? is the correct and natural form.