"Somali Govt" doesn't mean too much... The govt we recognize there doesn't control much of the country, and the pirates are big players in funding the islamist insurgency. Both sides claim that they are the legit govt. So far, the only nation who has been invited in to help in all ways with the piracy problem are the Russians, but they simply don't have the navy or funding for effective expeditionary forces.
The US and our other NATO allies could probably do it, but we weren't invited.
As for why the oil tanker managed to get caught, imagine the ecological disaster an RPG or two would cause... If I was a pirate I'd have one dude tasked to vent the entire tanker to sea if he saw or heard anything resembling a rescue or boarding party. The amount they're asking is less than what it would cost to clean up the mess if the oil was dumped, so the boat owners (or whoever contracted for the cargo) will probably just pay to avoid getting sued out of existence.
