In episode XII Jack and the gangsters, we learn of the Jewel of Neptune, a powerful item that is guarded by three elemental monsters (that I shall call Earth Wind and Fire). The reason Aku has not taken the Jewel of Neptune was that he was defeated by E, W, and F (or at least that he was unable to beat them). How is this possible? The only things that can harm Aku are Rama's arrows, Odin's Trident, Ra's energy beams and Jack's sword.
The best way to answer this in canon is to draw from what we see in Season 5. When Season 5 opens, we find that Jack is without his sword and Aku has a sort of... ennui about the whole thing. He's not even aware that Jack is swordless because he's gotten tired of the fight.
Let's apply that here. From the unofficial transcript (emphasis mine)
Long before you was born or I was born or anybody was born the Neptune Jewel was kept inside this mountain, see? Legend has it, it was placed there by an ancient spirit who gave the jewel special powers to control all the waters of the world, see? Naturally, Aku wanted the jewel's power so evil could control the world, which it should.
But then, this no-good, goody two-shoes, good-for-nothing got in the way, see? I don't know how she did it, but she beat our boss. And she summoned the forces of nature to guard it: Earth.
And so Earth, Wind and Fire have guarded the jewel for centuries, see? Making it impossible for Aku to get his hands on it.
And since the big boss himself couldn't close the deal it's been centuries since anybody's even thought about it.
So we know that Aku was not beaten by the three forces, but by whomever created or summoned them. And likely she made it just difficult enough for Aku that he couldn't easily overcome it. As a result, Aku just gave up on it.
In other words, those three never beat Aku, nor were they impossible to defeat (Jack beats them pretty handily), they just made it not worth his time to keep trying. Which is generally what good security systems do.