The census of the country is primarily the count of the people residing in the country at the moment the Census is organised. The birth and death rates of a district (place) will not give us the correct number of the people residing in that area.
Along with the count of the people, the census also does the calculations for various other factor like, the availability of public amenities, literacy rate of the place, etc. These details will not be provided by the birth and death rates.