When they first tried to come through, the child Gwyneth was chosen because she wasn't particularly bright. Well, yes ... she was clairvoyant, but that wasn't the whole reason. Nor was it her lack of education. Truthfully, she was somewhat simple-minded, even for a human being. Witness her chastising her employer for the re-animated corpses ... as if he could control a superior being! Simple minded and with very little in the way of what humans called ‘common sense.' And yet, for all that, she still managed to stop them. Not defeat them, no ... and she couldn't send them back. But she stopped them and held them in place ... until a new doorway opened up for them. It was quite mortifying to be bested, much less by a human being.
To their kind, time didn't have the same meaning as it did for humans. Thus, what was a matter of months, or even years, in the human world ... well, it wasn't like that for them. The hardest part of their new campaign in reaching their new outlet was getting past the Guardians of the Rift, but once more, a simple-minded human helped them. Evidently, she had a habit of making trouble, and not even her age-mate could keep her out of trouble all the time. Regardless, they were able to use her to bypass the Guardians ... or rather, distract them.
By the time the Guardians realized they were through the Rift and beyond their power, it would be entirely too late. And just as they did in Cardiff 1869, they chose a youngster with more power than intelligence. She was touched by the Rift, they realized quickly, another child of the Rift. Because long before the Rift in Cardiff was closed, this vein was just waiting for the right person to open it. It was a source of great, untapped power ... untouched and so rich. Now the Rift was open, and let humanity beware.
The Gelfth were returning, and this time, they intended to reap their full harvest. Humanity could not stand against them. Torchwood could not stand against them. The bodies of Earth were their reward for their patience, for what the Time War and the Time Lords did to them. But, to ensure that they would receive that which was their due, they had a secret weapon, something that would effectively neutralize Torchwood.
They weren't the first to make that mistake. Nor would they be the last.