My question is, when の/んです の/んだ refers to a situation or state of being, as per the following example:
クリスマスの前デパートはいつも込んでいるんです。Before Christmas, the department stores are crowded.
...why is it omitted in this example, which likewise describes a situation or state of being:?
お花見の時はどこへ行っても、人がいっぱいです。During cherry blossom viewing season, there are lots of people wherever you go.