The main reason why there are not a lot of left handed pro bowlers is because most people are right handed and the people that are left handed don’t bowl as a pro. Being a pro bowler is not easy and most pro bowlers are right handed. There are some left handed bowlers but some of them are not pro bowlers.
You are starting to see more and more two handed bowlers than left handed bowlers. Left handed bowlers have a great advantage over right handed bowlers since they don’t have anyone on their side of the lane and there is less traffic on their lane. The best right handed or the best two handed bowlers can be beaten by a left handed bowlers but there are not a lot of bowlers that bowls left handed these days.