Why no んです in お花見の時はどこへ行っても、人がいっぱいです?


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.




I think you did not refer to the right chapter of your textbook. If you have
already read the proper explanation about this の, you should know it's mainly
for explanation or clarification. Think of this の as a milder version of "that
is to say", "because", "I mean", etc. Your first sentence,
"クリスマスの前デパートはいつも込んでいるんです。", is not a natural sentence without a previous
context. It must be preceded by something that sets up a context, for example
"今日はデパートに行きたくありません。". Your example about お花見 is natural if it appeared at the
very beginning of an article.

Here are some links that will help you:

  * [What is the meaning of ～んです/～のだ/etc?](https://japanese.stackexchange.com/q/5398/5010)
  * [How is the "のです" working here?](https://japanese.stackexchange.com/q/3349/5010)
  * [Explanatory のだ (んだ)](https://www.wasabi-jpn.com/japanese-grammar/explanatory-noda/)

