In an old forum post he said he wanted to make it 42 as a hitchhiker's guide reference but didn't think people would take it seriously so cut that in half.

Interestingly the numbers work out really nicely with 21 million where the amount of coins distributed each block is the same as the percentage of the total coins distributed in that epoch. That's to say when 50 coins per block were distributed, 50% of the total supply was distributed by the end of that epoch.