Happy 2018 everyone! And I'm (a little) belatedly kicking it off with a question that may quite possibly be a duplicate, but I thought I'd risk it anyway. I quick searched it but there are so many instances of の/んです の/んだ here it would have taken me hours to go through them.
It's the first of several questions, all regarding the の/んです の/んだ construction. I've read everything my text books have to offer on the matter, but I'm still not managing to grasp certain particular uses — or in this case lack thereof — of it.
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.
Any help would be much appreciated.