As other people have noted, Somali piracy is not "new". It's been happening since the 90s (Somali Civil war and failed international interventions). There were, and still are multinational (basically chartered by the UN) naval task forces operating in the area, to deter and interdict pirates. See CTF-151 (https://combinedmaritimeforces.com/ctf-151-counter-piracy/)

These types of actions are not perfect, they cannot stop everything, so you still see successful attacks happen.

And no one wants to try to intervene in Somalia itself. The world tried that in the 90s and got completely burned.

So the answer is that "other countries are not allowing it" in the same way that no country allows murder, and yet it still happens.