its because not many schools have classes for all 5 subjects
normally there wont be enough students for that class, so they just cancel it (like my school this year)

you still can register for all the subjects, but you'll have to study 1 of them on your own as you'll be entered in a 4 subject class

IMO, only take more than 4 subjects if :
you're not sure what you'd wanna go into, so there's a wider choice after STPM
you're confident in scoring excellent, so why not add another A there to impress ppl?