Because duidelik is being used after the verb is.
In Afrikaans, adjectives often behave differently depending on where they appear:
So compare:
Die beweging is duidelik.
Here duidelik is a predicate adjective, so there is no -e.
die duidelike beweging
Here the adjective comes before the noun, so duidelik becomes duidelike.
This is a very common pattern in Afrikaans.
A rough guide is:
duidelik ≈ DOY-duh-lik
A few useful points:
You do not need a perfect accent immediately, but it helps to remember that ui is one of the characteristic Afrikaans vowel sounds.
A rough pronunciation guide is:
beweging ≈ buh-VAY-ghing or buh-VAY-ging, depending on accent and how strongly the g is pronounced.
Things to notice:
For many learners, getting close is good enough at first.
You would usually use the standard Afrikaans double negative:
Die beweging is nie duidelik nie.
Afrikaans commonly uses nie ... nie around the part being negated.
So:
The double nie is a key feature of standard Afrikaans.