Here's my wild guess:

For many years the 1488 and 1489 driver chips were pretty much what you 
used when you talked RS-232 - pretty much until maxim came out with the 
MAX-232.

These were inverting chips - it would not surprise me that the uarts 
were designed with the inverted polarity in mind.

-forrest

Wouter van Ooijen wrote:
> In a lesson I want to explain asynch serial data format and RS232 signal 
> levels.
>
> AFAIK the RS232 signal levels predate the 5V TTL levels (correct)?
>
> The TTL-RS232 level converters/shifters are all inverting. Does anyone 
> know why? Because inverting drivers are slightly easier to build? Or 
> were the asynch-on-TTL levels chosen first, and as a consequence the 
> drivers had to be inverters?
>
>   

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