Primary reason is that it's cheap practice, and you can introduce a new shooter to the AR platform easily. It's also quieter than centerfire.
Similar to the Ruger 45/22 or any of the millions of variants of 22LR 1911s.
Let me shoot 1,000 rounds of 22LR thru my dedicated 22 upper, and when I switch to my 5.56 upper with similar sights, it's second nature.
Similar to what up and coming motorcrossers are doing nowadays by racing 'mini-moto' bikes, and besides being slower and safer, it doesn't hurt as much when you come off of one of the little ones.