Because in "This peach is sweet" the subject is "This peach" and in "this is a sweet peach" the subject is "this".
But we had a sentence "This apple is sweet" that didn't habe הזה. We haven't had this yet. I unfortunately don't remember how the other sentence went
I entered the answer as “This is a sweet peach” for זה אפרסק מתוק It marked me wrong. I wrote down the answer supplied. “This peach is sweet” When the same question came up I referred to my notes and entered “This peach is sweet” It failed me and gave the correct answer as “This is a sweet peach” Not sure which should be used? Tested this elsewhere and it gave the first answer as correct.