Why isn't nunu ryze combo more popular?

I tried it out with friends, flex emerald elo. Even on a losing position, enemy couldn't do anything. Any stack, during sieges, baron, elder etc. got obliterated. Our winning moment was when all were dead except us 2, we tp'd to elder, nunu got penta and elder, we rushed it down and won. Isn't this a bit broken? Full build nunu can one shot anyone that is not a high MR tank, even that tank will be left with very low hp.