Should I use "why is this not" or "why is not this?"
Or are both correct?
The usual order is "Why is this not [ready yet]?" Inverting it to "Why is not this [rose in bloom]?" might be possible in poetry, but it sounds awkward at best in everyday usage.
Note: awkward at best is a euphemism for incorrect.
Edit: you didn't ask about it, but for completeness I thought I'd mention that "Why isn't this [all over the internet]?" is perfectly fine; indeed, it's probably the most common choice, despite the fact that expanding the contraction results in precisely the why is not this construction criticized above. (Contractions are like that.)
The "not" should be placed as close as possible to the thing it's negating; generally directly next to it is the most appropriate spot. Since you are not negating the entire "this", whatever it is, but only some element of it, the "not" should go next to the element being negated: "Why is this not blue like I asked for it to be?"
The following is coming from the perspective of a native German speaker.
As far as I understand English grammar, most constructions are strictly right-associative:
Why (is (this flower (not (on (the table))))), but on the floor?
Why (is (not (this flower (on (the table))))), but the other one?
It wasn't always this way. If you look at old English literature and poetry, the grammar shared a closer affinity with the German language, where the "not" is more flexibly positioned. (The "not" is left-associative in German, but not in the whole sentence.) In German, the important part of a sentence is always at the beginning, where the interrogative pronoun is placed.
For example, if the following question is asked:
Where must this stay?/ Where has this to stay?
Then one might answer:
Here / Not here … must this stay! / has this to stay!
This does sound OK, or at least not too odd in German, but I believe that this sounds really strange for native English speakers.
The position of the word "not" can be a bit flexible in newer English language, as in the above sentence, but modern English usage allows this flexibility less.
Please correct me, if I'm wrong. (Or if my English is wrong.)
I can't think of a rule but "Why is this not a good example?" sounds way better to me than "Why is not this a good example?"