Why Penguins Can't Fly?

Why penguins can't fly?

It is believed that evolution due to the lack of land predators made penguins change their wings for flippers.

This was also a consequence that penguins had to find food in the ocean and wings were difficult to use underwater, therefore they were changed to flippers, which greatly help penguins to swim and have made them extremely agile in water.
 
Besides, during this transition, penguins adapted further to life in water, changing their brittle bones needed for flying with strong, heavy bones which help them submerge in water.