Because I personally think a big part of why Amy did end up marrying Rory at the end without ruunning again, was because her parents were alive. Without her parents no wedding, therefore the Doctor's little trick wouldn't work.

Slight Difference

"Rose, I need to speak to Amy. Please go and get her," the Doctor stated, looking at Rose with heavy lidded, half-dead eyes. She now knew his plans, what he was about to do and, he knew in that instant that he shouldn't have told her.

Rose refused to move from her position and get Amy. Amy was his only hope for survival after the event, or at least, he was pretty sure she was. If he got lucky he might make it out of this alive, unscathed and on this side, but he doubted it.

It was his mistake if he never existed, because he now couldn't get to Amy.

"But, Doctor! These are your last moments alive. I need to spend them with you, not her. She loves Rory, not you! You don't need to speak to her."

He closed his aching eyes and shook his head slightly. "Rose, I may be able to survive this. But to do so, I need to speak to Amy."

He saw Rose clench her jaw in a way he knew meant that she wasn't budging from her spot. When it came to him, she never would. He should have left her, once again, on that beach. He knew something like this would eventually happen.

"You're only saying that to make me move. You just told me you're going to die from this. You can't have it both ways, Doctor. I'm not leaving."

He grinned at her, not able to stop himself, slowly lifted a hand to pat her cheek and let his hand fall heavily to his side. He was still very weak. "Oh, Rose. You're dooming yourself to have never met me. And now you never shall."

She blinked at him at that, confusion showing in her eyes. "What does that mean? Of course I know you, I love you."

He shook his head slightly and licked his lips. "Love doesn't matter in this case, Rose. I won't even be a figment of your imagination, like I will for Amy. I'll just never have existed. You'll be stuck at your dead end job, probably dead by Auton. If you even survive that long. Earth gets into a lot of scraps throughout its history that I have stopped. You might not even be born. Something of me needs to exist to save it, Rose, even a memory will do. And the only something I have is in Amy's head."

Anger was now in Rose's eyes, clear as day, and he knew there would be no saving anything. He would save the universe, only to lose the planet he cares the most for, and all the people he had known and loved.

He sighed in defeat. "Fine. It was nice to have known you, Rose. It was nice to have known anyone, really. Good bye. Please step back."

She nodded, gave him a watery smile, when she realised she won the battle and she was the last person to see him alive and as healthy as possible. He smiled sadly back, before turning his glance to Amy, sitting beside Rory, the two once again locked in a kiss that seemed to never end.

All he could hope for now was that the Earth could live without him.

He started the machine, let the doors close in front of him, and activated it to a crash course with the TARDIS.

The Doctor soon never existed.