If that was a design choice, why was it called Test::More

Because it tests more than Test::Simple. It's not Test::Everything :-)

Schwern (quite rightly in my opinion) has kept Test::More to a minimal set of useful stuff.

And incidentally, I've also wondered why there wasn't a not_ok function.

Probably because providing one wouldn't really give you any additional useful information over doing something like:

ok( not $expected_to_be_false  );
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