FM200 is not the only system available - Inergen, Novec 1230, CO2, etc (nobody has used Halon in ages if they can avoid it, as it's toxic.)
Gaseous fire suppression systems have numerous requirements that make them unsuited for a large publicly accessible space. There's oxygen displacement; most of them are "nontoxic" to breathe but still displace oxygen, so you have to have various measures to keep from killing people - that could range from delayed discharge up to SCBA stations (and staff training, maintenance, etc.)
The other problem is that you need sufficient concentration of the agent; the concentration varies, and some need higher concentrations (and better sealing) than others. That means quite a lot of work if the space/building wasn't built with it in mind. Even for a relatively small and simple server room, gaseous fire suppression installation is expensive and a general pain in the ass.
The systems are intended for spaces that aren't normally occupied. Vaults/storage for example, and industrial spaces (electrical substations, for example.)