Okay, first off Disney rarely sticks to the original story.
Disney's adaptation of Hercules is no exception.
In the Disney movie, Hades promised Hercules that Meg wouldn't get hurt, and in exchange for that promise Hades took Herc's demigod strength for a limited time. He was going to get it back either way. But when Meg was injured, Herc's powers flowed back into him, allowing him to lift the pillar off of her. But she was crushed and dying.
After defeating the Titans Hades reminded Herc of Meg at her last breath.
His last heroic act was diving into the bowels of Hedes to retrieve Meg. That was what made him a God. Meg had no chance of becoming a God, no matter how noble she was, and lets face it, she wasn't. She was in love, and a hero in a sense (since she put herself in harms way to return Herc's powers) but she wasn't the most noble or charismatic character. Not that it would have mattered if she was as greek Gods didn't care for most humans and their vendettas, no matter how illustrious their efforts.