Just to another detail to Blood's post.
Dark Danny was beaten by Fenton. Clockwork's influence made sure of that.
Now, Dark Danny's existence is in the control of Clockwork, technically, even if Dark Danny could take over, he'd still be at Clockwork's mercy.
Seeing as Clockwork had changed the event in Ultimate Enemy, so that the Ghost Zone and Human world would remain a more peaceful state, it wouldn't be surprising for Clockwork to also have a hand in Walker's development.
I remember the last time that questioned was asked.
Given the events of Ultimate Enemy, remember that the horrid future had to happen before he could change it. One option is that GT is the future he has yet to change, which in all honesty is a bit boring.
My theory, however, is that PPGD and GT is similar to Steins;Gate. Infinite timelines that all run parallel after their own points of divergence. In this theory, Clockwork can't just erase the events of any timeline he disapproves of. It's already implied that Clockwork can't just do as he pleases.
Clockwork wrote:I'm the Ghost of Time, not the Ghost of Miracle Cures.
To add on past that. The Observants from Danny Phantom also seem to not be able to view time in the same way Clockwork does.
I believe that the Observants posses an ability, Reading Steiner.
Reading Steiner is basically the ability to retain memories after a timeline has been altered; in another sense, they can travel to other timelines without losing their memories. However, the Observants seem incapable of making the changes in time necessary for their desired timeline. In Ultimate Enemy, they disliked the timeline where Dark Danny existed, so they 'commanded' Clockwork to kill Danny Fenton. Clockwork changed the timeline in which they resided, but without killing Danny. The Observants were annoyed with Clockwork's solution, but accepted the results regardless.
Dark Danny's existence is said to have been removed from time altogether. I believe this is merely a figure of speech, by 'time' they imply their current 'timeline'. This allows Clockwork to create contradictions in certain timelines, such as allowing Dark Danny's existence in GT. To paraphrase, one point of divergence would have been the event that ultimately led to Dark Danny's existence, for the sake of simplicity, we'll call this point A. From point A, multiple timelines are created that share his existence. Clockwork, instead of creating/moving to a timeline that diverged before point A, he created a timeline after. Stemming from point A, GT should have had Dark Danny, but didn't. Such an alteration allows Clockwork to 'fix' Dark Danny's existence.
Take what you will from this, it's all still a theory. c: