Why is everything modern if it has only been 70 years?
Modern? Doesn't look much past the 1920s or 30s technologically. But, for the sake of arguing:
1. They can BEND ELEMENTS! Making technological and infrastructure improvements much easier
2. Not everywhere is all that modern looking, like where Korra is from.
3. The Fire nation was pretty much in an industrial revolution during the original series. Our industrial revolution was from the early to mid 1800s, adding 70 years to that and you got... early 1900s.