I don't know but my guess is to add compatibility for RS232 voltage levels would add significant cost and complexity to the USB PHY, the thing that actually drives and measures voltages on the wire. USB was specifically designed to have a simple and cheap PHY so that it could be used in very cheap peripherals.
To expand a bit, the typical fully compliant RS232 setup has a special level translation IC (for example the MAX232) to deal with the huge voltage range and convert it to something more low voltage digital logic friendly. And that IC usually requires power supply levels that the typical cheap USB device would not have, which means it would have to add more circuits to generate them, adding even more cost to the hardware.
And the USB committee probably figured to be not fully compliant with RS232 would just confuse people, and damage hardware, so it was better to be not compliant at all.
It would have been really awesome if USB was compatible with TTL-level (5v, maybe 3.3v) "serial". This nonstandard variant of definitely-not-RS232 is everywhere.
But...
> And the USB committee probably figured to be not fully compliant with RS232 would just confuse people, and damage hardware, so it was better to be not compliant at all.
To expand a bit, the typical fully compliant RS232 setup has a special level translation IC (for example the MAX232) to deal with the huge voltage range and convert it to something more low voltage digital logic friendly. And that IC usually requires power supply levels that the typical cheap USB device would not have, which means it would have to add more circuits to generate them, adding even more cost to the hardware.
And the USB committee probably figured to be not fully compliant with RS232 would just confuse people, and damage hardware, so it was better to be not compliant at all.