Not sure what the right answer is.

But probably just because that's the total number when it starts with 50 BTC block reward and halving every 210,000 blocks for with 32 halving

So what I'm wondering are actually

1. Why block reward start at 50 BTC?

2. Why halving every 210,000 block?

On unrelated note. Why difficulty adjustment is every 2016 blocks?