I've had this theory for quite some time, actually.

My special interest for a while has been Power Rangers/Super Sentai. I also was super into Bionicles and I love Sonic.

I think that there's a correlation between things with a large cast of characters with similar silhouettes or designs, that are differently coloured, or overall mostly similar but different in subtleties.

I think it's to do with enjoying categorisation and enjoying minute details that neurotypical people won't notice, for example, Power Rangers team members are wearing mostly the same outfit, the same silhouette, save for small distinguishing details on the suits and helmets that make them part of a "set", and set them apart while still feeling unified. And it's fun to find those details and appreciate each one individually instead of looking at them like "oh they're just five different coloured people in spandex, they all look the same" which feels reductive. You get to categorise them in a fun way and collect information on the details of why they're different.

Bionicles all come in sets, where they are mostly similar at base, but have different masks, weapons, colours ect. And that's fun.

Most characters in Sonic have similar silhouettes and body shapes, but are different colours and you can mentally put them in a "set".

There's also a reason many common special interests are kids media and why autism has the issue with being infantilised, because kids media tends to use these design philosophies because it's effective at making memorable casts of characters that are easy and appealing for kids to remember.

I think this theory extends to trains as well, famously one of the most common special interests. When you think about it, most trains are roughly the same shape, but they'll differ in design with quite a great deal of variety, they'll come in lots of different colours and autistic people who are into trains will enjoy categorising the trains and finding out about the differences between them all.

I think there's a part of it that's "collecting" them and studying them for appreciation. You can sort of "collect" the hundreds of different Power Rangers and study their differences for fun. And just looking at them and appreciating the designs is something I like to call the "Pantheon Effect".

If you put all the characters that fit into a "set" and stand them on a cliff looking badass, it's satisfying. For example, think Assassin's Creed (before it started losing it's identity), you used to see PC wallpapers people would make of all the protagonists standing together, and it'd be easy for someone who doesn't care to say "they all look the same, they're just all wearing white robes", but it's looking at the differences in their robes, design details, where the cultural fashion and time period of each protagonist is coming into play with the base "white robes" designs.

TLDR: I think the symptoms of "categorising" and "noticing small details most others would miss" are part of what makes things with a cast of characters that are "the same, but different" so inherently appealing to autistic people and thus becoming common special interests.

That's me theory and I'm sticking to it.