I think most people (me included) took the very strong wording of the challenge (in a SIGNIFICANT way) combined with how strict many of the others have been and assumed it had to be like a core mechanic. I could have very well checked that box because most of my levels are kinda symmetric and balancing is a big part of my game, but I didn't feel like I earned it - which is kind of what they're there for, not boxes to check but hard things to design around and commit to if you want to.
Viewing post in Why did so few games take on the symmetry bonus challenge?
That was my interpretation too; though, I can definitely see how others would interpret it differently.
I've also seen a lot of people say this was their first jam so I think it's fair to assume many just weren't sure yet what they could handle in that timeframe and gravitated towards the more technical challenges, if any. Single-pack seems difficult at first, but some of the packs like the Kenney Space Kit and Kay Platformer kit practically have an entire game in their preview images already so I can see why people would be more drawn to it. Similarly, I can see mouse-only being challenging for a ton of genres but I noticed lots of people uploaded more management-heavy games which aren't as hard to design around not having keyboard/controller inputs (I also wonder if people were more drawn to that genre partially because many of the Kenney preview images look like management/RTS games).