It's an implementation issue.

I posted this as a comment to the article:

-- I'm working on a Ruby compiler and have taken pretty much [the PyPy] approach [of automatically using slots when possible]: Any instance variable names I can statically determine are candidates for the equivalent treatment (allocating a slot at a fixed offset in the object structure).

Anything else will still go in a dictionary. In practice my experience is that a huge proportion of objects will have a fairly static set of attributes, and the dynamic set is often small enough that having pointers them included in every instance is still often cheaper than using dictionaries. ---

In a static language, your options are generally to either statically allocate slots, or explicitly use a dictionary anyway.