It is very difficult to control human populations because procreation is something that people see as a fundamental right and it is something that is exceedingly hard for a government to control unless the government wishes to be extremely brutal and repressive.
Reproduction is something that is fundamental to human existence. The desire to procreate is strongly wired into most people. Perhaps because of this, there are many religions that feel that people must be free to have as many children as they want. What these things mean is that a government that wants to control population growth has to fight against some very deep-seated human desires. This helps to make it difficult to pursue population control.
In addition, it is very difficult to actually prevent people from procreating. A government would have to be willing to infringe very severely on the rights of its people if it were to control its population. It would have to engage in such policies as forced sterilization or forced abortions. Only the most repressive of governments would be willing to try to do these things to its population.
Thus, population control is “difficult to tackle” because it requires governments to take extreme measures to overcome basic human impulses.