Have you ever asked yourself this question? I've found it answered in April issue (261) of Circuit Cellar, in Test Your EQ section. The answer is quite intriguing.
First of all, DTL and TTL logic families were the most popular when RS232 was developed. In both DTL and TTL input current is significantly lower in the high state (a TTL input is equivalent to NPN transistor's emitter), so it makes perfect sense to use high state when idle to minimize the power consumption in the logic part.
The second part of the story is more funny. Why is high state represented by negative voltage on the line? This is because copper wires exposed to moisture are less likely to corrode if the voltage is negative. Indeed, it's the anode (positive electrode in electrolytic cell) where the oxidation happens. Beautiful explanation, isn't it?