Why is this?
- The Area is too wide, with no real confinements for the monsters, thus
-- A Player cannot separate their spawned monsters from someone elses in any way.
-- A Player will in fact get hindered by the other players monsters constantly
-- It gets hard to find a 'clear' spot after a while.
-- It is hard to act as a group, if after every death you have to relocate and Fly Wing all over to find even non-boss monsters you spawned.
-- People abandon their monsters a lot, leaving them to wander all over the map, again, getting in the way of others.
- There does not seem to be any NPCs, so
-- A player who spawned something they did not prepare for has to die to get access to a Kafra to get their equipment.
---- See point about relocating the monster.
What sort of map am I trying to suggest?
- One with NPCs, if only Kafra and Warper (as Healer could be abused) and maybe Merchant.
- One with separated areas, so
-- Several Players can spawn monsters without hindering each other.
-- Players can more easily relocate their monsters after deaths.
-- It is easier to find a 'clear' area.
I don't know what the map the server I previously played on used, but it's layout was somewhat like this:

A thing unrelated to the map itself, would be that maybe the map would wipe itself after a while, i.e. clear itself of mobs.
Problematic would be people getting their mobs wiped right after spawning them, though...