Left arm wrist spinners are an unusual breed.
Has there ever been one at international level who was wildly successful? I can't think of any off the top of my head, to be honest. Blokes like Bevan, Katich, Hogg and Paul Adams are recent examples, but none had great overall records- though Bevan and Katich did clean up in Tests a few times. Sobers bowled it on occasion, but I'm pretty sure he bowled much more left arm orthodox. And of course, he tended to bowl pace more than either type of spin anyway.
I dunno, perhaps it's got something to do with the majority of batsmen through history being right handed- really, because of the angle of delivery (whether over or around the wicket), a left arm wrist spinner isn't generally going to worry a good right hand batsman that much. Phone's pretty much on the money with what he said above.
Left arm wristies might threaten left handers more, much the same way as right arm off spinners (Swann being a recent example) often trouble left handers.
Let's think of all the possible types of spin bowling:
Right arm wrist spin (conventional leg spin)- plenty of those
Right arm finger spin (conventional off spin)- probably the most common type of spin bowled
Left arm orthodox- not at all uncommon
Left arm unorthodox- very uncommon, easily the least frequently bowled of the four.
Just one of cricket's curiosities, I suppose.