https://japanese.stackexchange.com/questions/1395/what-is-the-difference-between-the-nominalizers-こと-and-の

Both "のは" and "ことが" can be used to nominalize a verb, but they have slightly different nuances.

• "のは" is often used when the speaker wants to emphasize the action itself or the general concept of the action. It's commonly used in conversations and casual writing.
• "ことが" is more formal and emphasizes the abstract concept or idea of the action. It's often found in formal writing or when the speaker wants to sound more polite.

So, which one to use depends on the context and the level of formality you want to convey. In the sentence "挨拶するのは大切です," "のは" is used because it's a more casual context.