To be fair, it also costs a lot of money to do a census.
Have you seen how the Russian economy is doing lately?
Besides, that money would likely have to be carved out of the military budget at this point, since all the other government spending is down in the new budget to help pay for the massive increase in "defense" spending. Why would Putin want to spend money on a census when he could just use that money to buy more recruits for the meat grinder? One less vatnik to count next time, after all.