I'm not maining BRD and I have not even started MCH yet, but I do have a guess.... Maybe it's because they actually get booted from content, many times without a word or even a chance to prove themselves. Maybe it's because many people outright refuse to even take them to certain content - just today I saw a Bis Ex PF with "some jobs locked", ie. BRD and MCH could not join at all. And it was a practice/clear party, so not even one of those farms that require you to be at i300 in every job.
The core of the fault is probably the new tweaks/skills SE designed, but that's not to say people who choose to exclude them like that are entirely blameless.