Those two paragraphs sum up pretty much my entire grouping experience of the last 5 years in FFXIV.Dancer is popular because it's easy and fun. You can turn your brain off, just push the glowing buttons, and then watch the sparkles. No need to worry about positionals, little need to worry about your rotation, etc.
Because Black Mage is one of the hardest jobs to play. In order to maximize your damage, you have to stand still. If you move then that's a huge DPS loss, especially if you allow your stacks to lapse if you haven't casted a spell in a few seconds. If you stand still for too long trying to get a cast off and then die to an AoE, that's a huge DPS loss. You can only really familiarize yourself with the timing and placement of AoEs in boss fights after spending several hours progressing on them, so you only start getting it down and doing good towards the end of a prog run. For everything else, queuing for something and then landing a boss you hadn't done in a while sucks because you're just not going to remember where to stand and for how long and you're going to do little DPS, and that's not fun.