"This bus is slow, you know."
い-adjectives like 遅い act like verbs and are capable of conjugating and ending a sentence on their own. There is already a meaning of "is X" built into them so です here is mainly only to make it sound more polite. だ is a casual copula, and since the function of a copula isn't really needed here, and it also doesn't change the politeness level at all, it would be incorrect to use it.
The casual form of this sentence would just be このバスは遅いよ
So です can go after i-adjectives to make them more polite
but だ can only go after nouns and na-adjectives (which act like nouns)