IF he had a bounty. Remember the WG most probably don't give a f!ck about Skypiea. It's highly likely that very few members of the WG even know of it (Gorousei for example).
His bounty is given for nothing but strength...
Again, you're not bothered by your lack of knowledge to still give an opinion on this matter. Which is fine, but don't pose it as a fact because it's not and it's wrong.
oda wrote in an SBS that if Enel WOULD have a bounty it WOULD be 500.000.000 Beli.
And if there was a 500 Million bounty head in Skypiea, a place that at least some of the higher ranks should know about they would definitely intervene, they won't let such a big criminal roam free. Even Rayleigh said that he wasn't safe from the marines if they found out his location.
But that all doesn't matter because he didn't have a bounty. You should look up what Oda said on the bounty topic, bounties aren't entirely based on strength. That's why Caribou, Krieg and Bellamy had relatively high bounties for their place in the story.
And answer me this, If Enel was really worth 500 Million and if it was really based on strength alone, don't you think that He should've at least killed Luffy ?
Now get your arm out of Enel's ass because it isn't funny anymore.