Short honest answer?
Server stability was already proven to not be able to hold large amounts of players in a single area on a single mob. (Odin / Behemoth)
Average zone size isn't large enough to accommodate varied spawns and large mobs seeing as you can get from one end to the zone to another in virtually no time, it would help if the average zone size was around the size of Southern Thanalan or Coerthas.
World travel is too easy (cheap / fast) via Aetherites and party teleports making it so it takes literally no time for large numbers of people to overflow an area and zerg the spawns, keep in mind these zones AREN'T empty when people are teleporting to them. players are still doing FATEs / leveling / questing etc when these mobs spawn.
Mob damage / HP doesn't properly scale when large numbers of people rush and burn them down, it's likely these mobs were designed for groups of anywhere around 16-20 or so people for S rank not the 50 + that a lot of them are seeing.
The rewards are high which wouldn't be a problem if they were available from more sources (which they will be eventually) but for now with the ease of hunts compared to other venues it becomes the go to content encouraging more people to participate than the game can handle.
People try to liken this to FFXI's NM system but tend to forget that even in the hayday of teleports/outpost warps it still took considerably longer to get to any one NM / HNM when they were up unless you were in a group actively camping it during it's window.
In most cases you were sitting in a zone for hours waiting for it to pop during it's window and unless your entire group was there (HNMs mostly) holding the mob on pop with a small party while you waited for your LS to rush to help you.
They way hunts were advertised they were designed for small - mid group content, not something the entire server is intended to do simultaneously, it's likely that was more along the lines of what FATEs were probably meant to accomplish.