System.Text.Json isn't needed as a nuget library for newer development, the library is only needed for older stuff.

Those older stuff are likely running newtonsoft.Json anyway.

There's also a load of .NET Framework applications still running Newstonsoft.

System.Text.Json ships as part of the shared framework in recent versions of .NET so for most users it won't be restored from nuget.org

System.Text.Json is out-of-the-box in .NET > 5. The NuGet package is primarily a compatibility layer for people still supporting .NET 4.x for whatever reason.

> Why is there so much difference in the NuGet downloads between both libraries tho?

Because there's a boatload of older .NET apps that have been using Newtonsoft for over a decade already and aren't in a rush to switch. Anything built on .NET Framework is likely to still use Newtonsoft.