It's nice having seen these two get roughly equal love from fanart so far, instead of the female one just taking 95% of the attention as usual (especially since they're also split across versions and I'd hate to see version populations unbalanced entirely for waifu reasons). Pairing them up like this is also a cute idea.

I think one likely reason is that, while cute and attractive women are always popular, cute, somewhat effeminate young boys often also get a good amount of popularity. Both for wholesome and unwholesome reasons.
Another likely reason is that creepy yet cute characters are just an incredibly popular concept, like with Hex Maniac, gender of the character being inconsequential.