A practitioner of any martial art that has a black belt deserves great respect. However, I see all these dojos and gyms of non-BJJ martial arts, and they have multiple instructors. These instructors are all black belts, some of them having multiple/several degrees. At my BJJ gym, there are also several instructors, but only one of them is a black belt. I know that it takes longer to get a BJJ black belt, but not that much longer. Is there something that makes people quit BJJ sooner than other martial arts? If there wasn't, then why aren't there more black belts?
The answer is that it does take a lot longer for BJJ to reach black belt. Typically you can get a black belt in under 4 years for karate, taekwondo, and kung-fu styles. To get a black belt in BJJ, it takes about 10 years. It depends on how often you go, too, and how good of a teacher your instructor is.
Time on the mat is one criteria for BJJ rank advancement. Other criteria are: minimum age, ability to know and use required techniques for that rank, and the ability to win consistently in rolling against those in your rank and some wins against those in higher ranks. You have to have all of those, not just most.
Whereas something like Taekwondo may not have a minimum time in class requirement. Some do, most don't. So long as you've learned your forms, your single-step drills, terminology, techniques, sparring, and board breaking, then you're fine to test for the next rank.
Children are able to become black belts in TKD, not in BJJ. And in TKD, you don't generally have to be able to win in sparring consistently, so long as you're "okay" at it and have okay forms, one-steps, etc. In other words, you don't have to be good at everything, just 50% or more. The bar is definitely lower in most TKD schools than it is in BJJ schools.
And from a sales and marketing perspective, there's a reason why it takes just 4 years to become a black belt in those styles. It's because it's a goal that's within their reach. It's something they think they can achieve, and they like the status that "black belt" gives them. The longer it takes to achieve, the less people are going to sign up to begin with.
In Taekwondo, karate, and kung-fu styles, most people drop out after white belt, right after their short term (2-6 month) contract ends. They realize it's just not something they like doing. Then following that, you'll see a drop out around the 2-3 year mark, because that's when things shift from having fun to getting serious. Then the next drop out phase happens right after they get their black belt, because it was a goal that they achieved, and now they feel they've done enough and want to move on. They don't even stick around for their first stripe on their black belt.
Being able to stick to something for 10 years consistently is hard for most people. If you start as an adult at age 23, for example, that's age 23 to 33. During that period of time, you're starting your career, so it's expected you'll put in huge hours to learn that. Which means you might have to skip BJJ class a lot. Then you're dating, because you want to eventually get married. Dating sometimes means giving up nights and weekends at BJJ class. And then later on by age 30 or so, you're getting married. Soon after that, you're having your first kid, which takes 100% of your time for at least 3 months if not the entire year. And two years later, you'll have your second kid and will repeat your hiatus from BJJ just when you're getting back into it again after the first kid. You're getting older, too, and your body is not as quick and nimble as it was before.
Each of these things will cause interruptions in your BJJ training. Sometimes the interruptions mean skipping a class here and there. Other times it will mean taking a break from BJJ for several months or even years. That 10 year black belt time easily turns into 15 or 20 years. For most, it's unreachable.
But a 4 year black belt? Much easier to do.
What does take 4 years in BJJ is a blue belt. Just consider blue belt in BJJ to be "equivalent" to a black belt in most other arts. Then do your comparison. That's more apples-to-apples. I think most people suggest doing that, also.
Hope that helps.