Couple of issues.
1) I think a number of MMA fans (not all obviously) have biases against boxing because well, they buy into this competition between MMA and boxing and feel a need to be loyal to the sport. These guys tend to denigrate boxing and boxers at all cost and revel in the thought that elite boxers would get destroyed in the octagon by the elite MMA athletes (which under the MMA rules they would).
2) The best base argument is biased against boxing since elite level boxers don't participate in the MMA. On the other hand, the elite level wrestlers have no means to make big money besides the MMA and thus you get to see more of the best of the best wrestlers in the MMA rather than the boxers.
1) I think a number of MMA fans (not all obviously) have biases against boxing because well, they buy into this competition between MMA and boxing and feel a need to be loyal to the sport. These guys tend to denigrate boxing and boxers at all cost and revel in the thought that elite boxers would get destroyed in the octagon by the elite MMA athletes (which under the MMA rules they would).
2) The best base argument is biased against boxing since elite level boxers don't participate in the MMA. On the other hand, the elite level wrestlers have no means to make big money besides the MMA and thus you get to see more of the best of the best wrestlers in the MMA rather than the boxers.