Because RTD didn't go far enough!
I think RTD failed as a writer because his 'dark and edgy' sci-fi show 'Torchwood -- Chidren of Earth' actually had surviving characters at the end! What kind of wimpy writing is that?! After all, as the 'star' of the show Eve Myles said,
"We can’t mollycoddle the fans, treat them like children and say everything’s going to be alright."
So here are the missing scenes that should have been filmed:
1) Jack and Ianto drove off to Thames House, not knowing that Ianto would meet his death there. Rhys rushed off with Lois's incriminating footage to wait for further instructions from Gwen. In the meantime, Gwen and Clem waited at the warehouse for Agent Johnson to show up with her armed posse.
"Stay in plain sight, and place your hands where they can see them." Ianto had cautioned Gwen before leaving with Jack.
Gwen had smiled condescendingly. She had worked as a PC before, she knew how to handle it, thank you very much.
Agent Johnson and her men fanned out across the warehouse. They saw Gwen and Clem seated at a desk. As she sensed them drawing nearer, Gwen folded her arms, swung back her shiny hair and turned around to face the armed men, forming a sexy pout on her face. The sudden motion triggered a sharp command from Johnson and all the men opened fire on Gwen, riddling her with bullet holes. She fell in a heap on the floor, dying and realising at the last minute that she had completely forgotten to give any instructions to poor mentally-disburbed Clem, who was now leaping and whooping, thereby earning a barrage of fatal gunshots for himself.
2) As he was getting away from the warehouse with the footage of the politicians, Rhys forgot to look both ways whilst crossing the highway and was run over and killed by a lorry.
3) Jack woke up on Floor 13 to realise that his bravado and lack of planning had caused Ianto to die pointlessly. He was so overcome with shame that he decided to sacrifice his grandchild Stephen to the 456. Alice would have none of it but Jack was determined to get Stephen. So Alice slit the throat of Stephen and then slit her own throat so that Jack could not get either of them alive.
4) With nobody to sacrifice and no obvious way to stop the 456, the government had to try and round up the children.
Rhiannon and Jonny, with PC Andy's help, put up a spirited resistance to the police, but the police opened fire on them, killing everyone in the council estate, including the smallest child.
5) The UK government was able to muster the required number of children from the UK, but many countries with chaotic governments, such as Somalia and Afghanistan, just could not be bothered, so the 456 were not happy with the total count.
They released their super-deadly alien virus into the atmosphere and everyone on earth died. Well, everyone but Jack, who then started looking for a way to get out of earth because, man, the stench of corpses was just too much to bear.
----------------------------------------------- THE END ---------------------------------------------------