Q: version 3.0 removes categories and tables from  version 2.3. Why?

Numbers 3.0 hasn't removed anything from Numbers 2.3.  You've still got Numbers 2.3, intact, on your Mac and you can continue to use it, if you want, at the same time as Numbers 3.0.  Numbers 3.0 is in your Applications folder and Numbers 2.3 is in a subfolder within the Applications folder named iWork '09 or similar.

As to why Numbers 3.0 has fewer features than 2.3 none of us here can really say how Apple made its software engineering and design decisions, except that we know there is a push toward being able to use Numbers not just on the Mac but on iOS and on the web.  That resulted in compromises that make some people unhappy and other people a little happier.

You can read more here in the discussions about lost features and gained features. There is also an interesting outside article plus Apple's recent announcement about plans to restore/add features in upcoming releases. And some workarounds you can use for the time being.

SG