From https://wordpress.org/about/philosophy/:

Many end users of WordPress are non-technically minded.

The rule of thumb is that the core should provide features that 80% or more of end users will actually appreciate and use. If the next version of WordPress comes with a feature that the majority of users immediately want to turn off, or think they’ll never use, then we’ve blown it. If we stick to the 80% principle then this should never happen.

Different people have different needs, and having the sheer number of quality WordPress plugins and themes allows users to customize their installations to their taste.

It might only be a "rule of thumb", but I don't see how a CSS Editor is something that a majority of people would care to use. Where's the evidence to support this?