Because it depends what they are talking about.... If you get my meaning....
Dit is wrong here :S ?
det hvad hun sagde
"Yours is biggest." I don't know what English natives think about the superlative being used without 'the', but it does not sound right to me.
Hvorfor "den" og ikke "det"?
It could be dit if the speaker is talking about a -t noun. If the noun is unknown the default would be din
It could be a t-word, I guess. Like "Your house is biggest": "Dit hus er størst".