Why not every building just uses the simple, fast, cheap wet pipe design.
A wet pipe sprinkler system is the simplest, fastest, and least expensive automatic sprinkler design there is: the pipes are already full of pressurized water, all the way up to every head, so when one head's thermal element trips there is nothing left to do but discharge. If that's so good, why do dry pipe and preaction systems exist at all? Because "fastest" isn't the only design requirement in every building. Some spaces can freeze. Some contents can't tolerate an accidental soaking. NFPA 13 gives designers two other system types precisely to solve those two different problems — at the cost of a real delay between trip and discharge.