Here's why:
* Male Bettas are the sole caregivers. They build a bubble nest and guard the eggs, but they don't actively feed or protect the fry (baby fish) after they hatch.
* The fry are on their own. They are independent from the moment they hatch and must find their own food.
* Bettas are actually cannibalistic. The male betta may even eat his own fry if he gets hungry.
If you want to raise betta fry, you need to remove the fry from the male betta's care and provide them with a separate tank and appropriate food.