it has a small number of planets
This is because of EaW's freeze issue/selection bug. Basically, after a certain amount of stuff has happened in a GC, the game stops being able to track objects and makes it impossible to select units, so the game becomes entirely unplayable. Number of planets in a GC is pretty much the main variable we had under our control to mitigate this issue. Yes, there are other bigger GCs, however the fact that you start with only one planet means that each planet changes hands multiple more times in a GC before you even get to what would be the starting point of another GC. Its current size is basically right on the cusp of keeping it finishable before the error would occur.
There's nothing any mods can do within the game to fix this issue, but since the development of this map, we've been able to create a utility when can defreeze individual save games after it happens. This is useful for campaigns that stretch on longer, but we ideally don't want to create campaigns which rely on the user having to/being able to leave the game to run a separate utility multiple times to finish them at all.
doesn't even include Coruscant
The map was essentially set up to have different levels of planetary importance at equidistant jumps from each of the starting locations, and Byss was simply a better choice for this. Coruscant is certainly important to the narrative of this time period and any other time period really, but so are a ton of other planets, and there are plenty of maps which are focused on those narrative threads which already include it. Being a scenario which is completely agnostic to the greater story of the time period, it just doesn't really gain anything from having Coruscant in it.