But constant doesn't mean you can't change it in Ruby... keywords are basically the only thing you can't change. And most of those are not capitalized. true is a literal, just like nil or 250. You can't change those either.
Ease of use and conventions always outweigh consistency and simplicity in Ruby.
In common, all uppercase names are used for constants, except for classes and modules.
So True doesn't seem like an ordinary constant, and it doesn't feel nice for me.
Why there is TRUE is the historical reason.
In very early ruby, there wasn't the keyword "true", but only the constant "TRUE".