1) It's practical - for our team, we had developed in it before and had deployed a large system in it - we could get to where we wanted quickly.
2) we think it's a competitive advantage (see OP and PG's Beating the Averages essay) - additionally, since we deal with large amounts of data, OCaml was particularly helpful in that regard too
3) OCaml is a good filter - the thing that matters more to us is a person's elasticity of learning - their ability to learn new things. We use the right tool for the right job, so we also use javascript, java, objective-C, etc. OCaml is a pretty good filter and test for elasticity of learning.
Lastly, not a primary reason, but an advantage - languages get popular because the companies that use them are popular (i.e. gain market traction and can pay developers). That doesn't mean it's the most efficient way to get things done. We'd like to be part of the group that helps make the industry more efficient.