Bombing al-Shabaab is killing lots of its fighters, but is not hampering its ability to fight.

It is typical for guerrilla groups that killing many of their fighters has no effect, because they can easily get as many as they want. In the past this was usually because they were successful only based on popular support. In the case of al-Shabaab, it seems to be due to conscription.

If the people in the areas they rule get fed up with al-Shabaab's high taxes and conscription, they might be willing to support the government.