There are two statements that are unreasonable, and two that are perfectly reasonable.
This is unreasonable because MCH still fits their role as the Ranged DPS, with a vulnerability up debuff even. You're not the only one though. BLMs and SMNs usually get the short end of the stick as well in a minority of groups; they just want RDM for its mobility and utility, while some prefer BRD for its superior supportive abilities.
This is reasonable because another Ranged DPS has taken the slot. It is possible that an MCH had taken the spot as well. In a raid, we would desire only one Ranged DPS, so look for a group that has a free ranged DPS slot. When the above happens, just keep looking.