Because pseudo-ephedrine is a controlled substance, and the reason for that is that it is the chemical precursor in the manufacture of methamphetamine (crystal meth).
Its sold over the counter so apparently its not "controlled". Other "controlled" substances that are available over the counter are are Potassium permanganate, Acetone (nail polish remover), Hydrochloric acid (Toilet cleaner) etc..
Ammonium nitrate is used in many common fertilizers, but it is illegal to possess it on your person or to transport it or sell it, because in the right hands (or rather, wrong hands) it can be made into explosives. In other words, its use is strictly controlled, because of the potential illegal stuff that can be made from it. Same thing with pseudo-ephedrine.
There are more than 21,000 ammonium nitrate manufacturers in India. How controlled do you think it is ?
PS: It was made controlled only in 2011.