He had shattered the Lannister host, The Royal Army had barely held against Stannis and given the Tyrells have kept out of the fighting since Stannis killed Renly and they oppose Lannister rule nearly universally the only House in the way that could stop them was neutral and lacked a canidate to the throne.
._. So all that was standing in his way to King's Landing? Minor reistance and distance.
He could have taken the entire realm if he had pushed for it.
But see, how does that work exactly?
You capture King's Landing yes, a formidable hold... but then, why does that automatically make you the winner?
If everyone else is against you (and I'm talking about the entire South now) you're just isolating yourself from supplies. Basically, you've cut yourself from the entire world.
Unless I'm missing something.
P.S I found something in tumbrl that's hilarious, I'll post it on the Stannis thread xD