Doctor: Mrs Moore, would you care to accompany me into the cooling tunnels?
Mrs Moore: How could I refuse an offer of cooling tunnels?
Lumic: What is your name?
Doctor: I'm the Doctor.
Lumic: A redundant title. Doctors need not exist. Cybermen never sicken.
Doctor: Yeah, but that's it! That's exactly the point! Oh, Lumic, you're a clever man...I'd call you a genius, except I'm in the room. But everything you've invented, you did to fight your sickness. And that's brilliant. That is so human. But once you get rid of sickness and mortality, then what's there to strive for? Eh? The Cybermen won't advance. You'll just stop! You'll stay like this forever. A metal Earth with metal men and metal thoughts. Lacking the one thing that makes this planet so alive. People. Ordinary, stupid, brilliant people.
Chosen answer: 1) When Stephen Moffat took over he ignored a lot of what had been developed before (there is not in-universe answer). 2) It would have killed Rose, so the Doctor absorbed the energy. His body regenerated before the energy could do a significant amount of damage that would prevent regeneration.