It's because once the egg is fertilized it gets deactivated and can't be fertilized again For 2 or more to fertilize at same time, they've to arrive at roughly

It's because once the egg is fertilized it gets deactivated and can't be fertilized again For 2 or more to fertilize at same time, they've to arrive at roughly same time before the deactivation completes So this becomes rare because the window of deactivation is quite short and not many make their way to egg.