Why the package manager not show the newer ML-Agents package?

The latest of ML-Agents is 3.0.0: ML-Agents Overview | ML Agents | 3.0.0.

Why the package manager show me the older one?

Read the page you linked.

To add the ML-Agents package to a Unity project:

  • Create a new Unity project with Unity 2023.2 (or later) or open an existing one.
  • To open the Package Manager, navigate to Window > Package Manager.
  • Click + and select Add package by name…
  • Enter com.unity.ml-agents *Click Add to add the package to your project.

I know that, but why it show me the older?

In case you need the older one, which was listed in the package manager.

Why it not show the oldest such as 1.0.0? Why 2.0.1?

They probably only show up in older versions of the editor.