Let me tell you, I know exactly what you're feeling because I used to feel that way, too! If these people are doing fan art for these other comics, without ever being asked -- then what does that say about them not doing the same for mine? It's a slippery slope leading down to madness.
I mean, MAYBE that inner dark voice is right. Maybe those people don't like my work as much as they like the other comics. But what am I going to do about it? Other than continuing to make the best comic I can? If my best is still not good enough... that's rough (it really is), but I gotta deal with it.
But there are also other factors. I look at why I'm more likely to do fanart for X, but not for Y:
- I like X better
- I like Y better, but Y is so good that there's nothing I could add by making a fanart.
- Y is still in that mystery buildup stage/ I don't feel like I understand the characters well enough to capture their essence.
- It's much harder to find references for Y. (The comic itself is honestly not the best reference... it works, but not ideal)
- The creator of X seems much more enthusiastic when they receive fanart. If I like X and Y equally, I'd rather do something that makes someone's day than something that makes someone kinda sorta happy. I realize some people are just not as expressive as others, but it helps if you can show how much you appreciate receiving.
- Related: if YOU make fanart for other webcomics, people will see that fanart is something meaningful and positive to you.
- I'm friends with the creator of X, but not with the creator of Y.
- Many other reasons.
You could address all of those factors (providing easy refs, shouting from your rooftop about fanarts you've received, etc) and still not see any results. Luck is always going to be involved. It sucks for the less lucky of us, but all we can do is keep on trucking.
Keep on trucking.