Why DataDog is not in the S&P 500 even if market cap is 20B?

I thought that the S&P 500 included all the US companies ordered by market cap (and taking only the top 500).

So why DataDog for example is not included? It has 20B market cap, which is much higher than some other companies in the S&P 500 (some are only 3B).

What are the rules?