Adding to what ty said, being among the least played jobs does not equal "least enjoyed" and let alone "isn't fun/ doesn't flow nice" - that is only your opinion of the job. Not every job has to be for you but that doesn't mean it automatically lacks "flow" or isn't fun.
I played GNB at max level and even in some Ex trials and savage raids. So technically, I'm part of the crowd that plays it - so since I play it, I probably enjoy it, right?
No, not really. It's alright. It's the tank that suits me most but can't say I enjoy it to the point I'd play it more than once in a blue moon. Nonetheless, the data counts me as a +1 for "people who play and probably enjoy GNB". It's important to learn the difference between x amount of players playing a class and x amount of players enjoying a class.

BLM isn't the kind of class that someone who doesn't want to play BLM might still play it.
Healers and tanks have the bonus of having shorter queue times so there's an additional incentive to play them at least occasionally even if you don't enjoy it and I can guarantee you that a non-significant amount of people does exactly that. DNC is another good example of a class people are most likely to play even if they don't really enjoy it in the sense of playing it more than once in a blue moon. It's great if you want full mobility, fairly easy gameplay or just turn your brain off for a bit, push glowing buttons and still perform decently well. It's easy to play even if it's not for you. It certainly isn't for me on a regular basis but alright every once in a while. So I'm another +1 there even though I don't really play it.

So that means unlike many other classes, BLM player count isn't skewed by a certain amount of "I don't really like it but play it anyway" players; people enjoy it and play it or don't and not play it. So it's safe to say that the vast majority counting towards BLM players are people who genuinely enjoy it. Not every class has to be popular.
We really need to get rid of the notion that a class design is bad or unfun just because the class isn't topping the charts of people playing it. The fact that BLM has historically been the most consistently well-received class speaks for itself.
BLM simply isn't the type of "I don't really like it but play it anyway" class. And that is fine.