Tell me what is the difference between a pediatrician and neo natologist:

A  pediatricians is a specialist of children starting from birth till 18years. Some take them to 15 years.  Neonatologist,on the other hand is a sub-specialist of pediatrics who looks after babies from birth till 28 days of life.  They also look after premature babies. 

Why can a pediatrician not fulfill the need?

As I mentioned this is a sub-specialty of pediatrics and earlier pediatricians looked after neonates (newborns).  But it was realized that newborns make a major bulk of pediatric workload and need to be better managed.  Neonatology was thus introduced in 1960.  This filed like any other field of pediatric sub-specialists requires a lot of dedicated work.  In USA, a 1 kg infant born in 1960 had a mortality risk of 95 percent but had a 95 percent probability of survival by 2000 which came to around 98 percent by 2010.