My understanding (I'm a doctor but NOT an expert in this area) is that it usually is a long, difficult journey to the egg, and the chance of many getting there at exactly the same time is low. When a spermatozoan buries into the egg a reaction is caused which prevents other sperm from being able to penetrate.
An expert may be able to clarify, but I think this is the gist of it.
An expert may be able to clarify, but I think this is the gist of it.