I think it is because it is a combination of steampunk and cyberpunk.  There are robot soldiers that guard Shinra Corporation assets and the infantrymen wear advanced headgear.   The city of Midgar is buildt around giant Mako reactors that distribute energy to the city using large pipes.  The city sectors look vibrant complete with neon and yet it is interleaved with spaces that look like they came out of the Victorian era complete with clock towers and train stations.  There are large propeller driven airships and 1950's era style cars for on the road so there is a mish mash of even more things but the main setup is a combination of steampunk and cyberpunk.  The steampunk lends its atmosphere of historic feeling and nostalgia which is in tune with a Final Fantasy type story revolving around central characters identifying to past events.  The cyberpunk lends to its atmosphere on the level of foreboding and film noir/mystery.