Well it comes from inner intricacies of the metrics used to compare jobs within their own subrole, within which they do compete against each other. BRD and DNC are heavily slanted toward raid damage buffs that amount for close to a quarter of their total rDPS/cDPS contribution. This means that they're going to be very dependent of their team to perform optimally, and that's also why even before the recent buffs MCH wasn't doing that bad in lower percentiles.
On the other hand at higher level MCH cannot follow because MCH doesn't have the pDPS output to keep up with melees or BLM/PCT, simply because having a lower pDPS output than those, it will mathematically not benefit as much as them from raid damage buffs and play as well within those than them. You also don't dance partner a MCH usually for the same reason.
This turns it at low level or percentiles as a valued job that can carry its own weight, especially light parties and even more in solo (notably with a huge upfront damage), but at a higher percentiles that's lacking because it's not working within the same parameters as BRD and DNC as a heavy raid buffer. Well, that is, before the recent buffs, which definitely put BRD and DNC behind even at high percentiles, because rphys as a subrole is meant to suck alongside SMN/RDM, that's about it.