I'm probably going to end up oversharing so sorry in advance lol, but it's hard to say, honestly. 

Graphic design isn't as challenging in most cases (though it can be), and it pays more on account of being a less competitive field. In a lot of ways it's more of a science than an art; I'm neurodivergent, and like my friend puts it, can't tolerate ambiguity as a result, and in that regard that's a huge plus. It takes less time to do on average too, and it's easier for me to wrap up graphic design projects than others. At the same time, it's probably because it doesn't take a lot out of me to design something that I conversely never get to put much of myself into it. I have a very visual imagination, but a cinematic one, so graphic design is one of the last things on my mind when there's something I feel the need to express. I can't really picture myself firing up Illustrator or InkScape unless I have to.

While I get more out of illustration, generally, I haven't felt fulfilled in it in a long time. It's not really because I don't think my work is good, or that other people don't, at least on a technical level. I think a lot of it comes down to the fact that I see my personal work as a vector for my interests, and I want it to get more people into the things I like, so the subjects I focus on are very niche.
But even if I feel it's a good concept well-executed, I just think if it doesn't evoke anything more than that in me, then it won't evoke anything in other people, and that such things aren't really worth completing or sharing if they won't contribute to my "goal" of getting others interested in (or at least getting them to understand) what I like. I've realized my portfolio and social media presence has suffered from this approach, though, so I've been trying to do what I can and get past it.