The first ten or so levels are the slowest due to having to teach all the radicals that lay the groundwork for future levels, so the speed may be discouraging or frustrating for some that they might look for another (faster) resource or just simply give up. They may also just find that WaniKani’s teaching style or review system isn’t for them. Interests may also shift, or people may find they just don’t have the time. There’s a lot of factors at play, so the early levels end up sort of weeding out the people who don’t or can’t have the commitment to do this for multiple years.
If they’re going at a steady pace, this is probably around when enlighteneds and burns start piling up, effectively almost doubling the workload. People who do or did tons or all of their lessons at once are especially susceptible to getting overwhelmed. Review counts start to peak here and become a new standard, so it becomes another test of commitment.
If I recall my personal experience right, the level 20 area is when the kanji themselves start getting harder. There’s less radicals, but the kanji gets more complex now that you know the “main” radicals and mnemonics and are past the easier and more distinct early elementary material.
While I didn’t quit at any point, this reflects how I felt/feel at these points. In the 30s you have the vocabulary to start picking up more intermediate material without looking up every other word, and start to train in other areas of the language. In the 40s I was reading native material pretty fluidly, often picking up bits of kanji and vocab that is taught in later levels. And now in the 50s, I feel very ready to be “done”. I could very well stop here and start putting the time WaniKani takes into consuming more advanced native material and studying the upper levels of other parts of the language without missing too much. The last ten levels feel like bonus material, which makes sense since 50 was the original end point and they were added later in the program’s lifespan.
So yes, the first ten levels tend to make people stop and think about what they want and whether they want to continue, as does a number of other places. But as long as you pace yourself well, you have the time, and you find that WaniKani truly is for you, you may not even feel that big of a hump.
Another piece of it might be that I look even taller than 6’, probably because my legs are about 94cm long and I’m not nearly as broad as the average man, so I just look very vertical. My brother is 6’3” but looks shorter than me if we’re not standing side by side. I also look taller than my husband until we’re next to each other. 
It is whatever it is but definitely leaves me navigating a very wide range of responses.
It’s not like I go into hysterics every time someone brings it up or else I would be a hermit. Usually I just review my past involvement in sports, give a review of the weather up here, and move on with my life. But that’s a social situation I can navigate in English. It’s quite another to navigate it in a language I’m not familiar with with people who very well may have never in their lives seen a 6’ woman.