We spent a long time on the choice. But it was really important in this setting to avoid the word "type", and sort is a technically correct term too — e.g., as used in many-sorted logics [https://en.wikipedia.org/wiki/Many-sorted_logic]. Given the precedent from mathematics, it seemed like a pretty good choice (and to me, still does).
Because we didn't want to limit people's thinking to current type systems.
In my experience, people who have only been exposed to rather weak type systems tend to have strong (negative) beliefs about what types supposedly can and can't do — witness the number of pointless "debates" about "typed" vs "dynamic" languages. We therefore expressly wanted people to shed their baggage and think about what programmers want to be able to express and work from there, not see something hard or unfamiliar and say "oh, «types» can't do this".
Picking a term that keeps harkening back to that baggage would then be counter-productive.
