Literally just ease of use. Blender you have to learn to use, whereas JSM is built to be pick-up-and-go for even the least tech-savvy users.
And feature-wise, that's all it needs.
Try get a non technical artist to pick up Blender and work out a rigged human model.
first question why: where do you get a rigged human models with the same level of detail, for free ?
haha this was mine as well, would love to see some recommendations where people are getting some nice models to draw from.
I wanted to use blender for quick pose reference yesterday and I was able to find several good ones and even one great one within 5min. They're around.
Ease of use.
Because of a lack of features? It doesn't even have IK so I would argue Blender is in fact easier to use (as in, will get you to the result faster even if you need a bit more time to learn the interface).
This has pre-built poses, props and a bunch of other easy to use features. This is classic hacker-news ‘Less intuitive and more complicated X is better because I already know how to use X’
I must have spent a hundred hours in Blender over the years, and I'm still not competent enough to do even some of the most basic things on my own when I pick it up.
Blender is an amazing project, but I would not suggest it as an alternative to OP's tool if asked by someone who practices drawing and has zero Blender experience or interest.