Why not אש?
It's hard to explain. אש is used when the fire is under control, and שריפה is used when it isn't and usually you don't want it to happen.. I'll give you an example:
יש שריפה במטבח
חם לי בגלל האש
Is it incorrect to say מתי השריפה התחילה?
Same problem here
למה אין פה "את"
Because fire in this sentence is the subject, not the object. את is used only before the direct object.
Matai hitchila hasrefa?
I wrote it exactly as it appears in the answer, and they marked it wrong!