Because the current way isn't really a level playing field and people do have advantages over someone else, unless you are only counting people going from A to B, then sure, everyone on that flight has a level playing field to check in 24 hours before flight. Even.
But, if you consider the BIG picture, it's far from a level playing field for everyone systemwide. People leaving on a flight from MCO to DFW are even, people leaving from FLL to DFW are even, even TPA to DFW are even, but when you throw in everyone from these cities trying to connect to LAX, then it becomes a non level playing field. If the MCO flight leaves at 6am, FLL at 605 and TPA at 610, then there is no way that the FLL or TPA people (not to mention the poor DFW people) have a level playing field as far as being able to check in for the DFW-LAX flight on a level playing field 24 hours ahead of flight time because they're already penalized beacuse of where they are coming from and what times the flights are scheduled from that city.
FCFS might work great and it's easy to know where you stand on the list which is great and fine, but don't keep trying to convince me that it is fair and a level playing field for everyone, because the way it's being explained is far from fair for everyone. You might not like DOH, and that's understandable if you don't think that it's fair for everyone because not everyone has the same seniority, but I don't see that FCFS is fair across the board either as far as everyone is trying to sell it.