Well, the first UART I was associated with would be the mechanical stuff in
the teleprinters.  Later, after they "invented computers", I build myself a
"serial to parallel converter" (from TTL) to make my TV Typewriter work with
a modem.   Subsequently, I was at a hamfest (swapmeet) and a guy had some
40-pin parts which he described as apparently the same thing.  My eyes grew
wide and I purchased one of them, took it home, replaced my board with
that...It was a TMS6011 if I recall correctly, later discovering the more
"standard" AY-5-1013 from General Instruments.
So, these seem to have been the "first" UARTs.  And it never seemed strange
to me that 0V was considered the "active" state.  Not just because "mark"
was "current" and all that, but I had gotten used to CS# being low active,
MCLR# being low active, and so on.  It just seemed normal at the time.
Keep in mind that the TTL interface was never intended to relate
electrically to the +/- 12V one.  That was the converter's job, period.
 This apparent "UART over TTL" idea really was just the result of some folks
who saw a conversion shortcut they could make using a diode and a resistor.
Barry
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