Originally the Worldstone hid Sanctuary. Then the Nephalum were born into Sanctuary by the union of demons and angels, and in order to ensure that the Nephalum wouldn't draw the attention of the Heavens and Hells, Inarius tweaked the Worldstone to sap their powers with each subsequent generation. It is worth noting that the Angels and Demons saw that the Nephalum had the power to become greater and more powerful than either Angels or Demons. The Angels saw this as a threat, while the Demons saw this as a potential army.
Eventually Sanctuary was discovered and the events of the Sin War detail how eventually a peace accord was reached between the Heavens and Hells, and part of this meant that both sides would leave Sanctuary alone. All the while the Worldstone would continue to keep the Nephalum weak. Many of the Angels believed the Nephalum were "Abominations" but some, like Tyrael, saw that despite the potential for evil in the Nephalum, they also possessed the potential for goodness and virtue.
Diablo and the Prime Evils hatched a plan to Corrupt the Worldstone. They found a means of acquiring shards of it, and over the span of about 250 years, each of them managed to attune themselves to the shards. They would then use the shards to corrupt the Worldstone and turn all the Nephalum (humans) evil, remove the restrictions on their power, and then destroy heaven with their new army. Diablo's essence would imbue humanity with the desire to spread Terror, Mephisto's stone would imbue them with endless Hatred, and Baal's stone would imbue them with the desire to Destroy.
The plan failed.
The Worldstone was corrupted by the Soulstone of Destruction, but the Worldstone was destroyed by Tyrael before the evil influence could spread to humanity.
Hell could have invaded before, but it knew Heaven would reply in kind, and that would simply have continued a stalemated war. What Hell was truly after was an army of Nephalum to tip the scales of the war. At the end of D2, Hell was denied that. As such, just because Hell could invade Sanctuary, doesn't mean it was advantageous to do so, which is why it hasn't really happened... until now.
Because now Azmodan has found a reason to go to Sanctuary. A Black Soulstone with the power to imprison ANY Angel or Demon. A Power that could tip the scales once more..