I originally joined Reddit in 2011, and in the beginning it was dominated by cat content. It was rare to see a post about dogs. Sometime in the mid 20-teens this phenomenon reversed. It’s always been a “pet theory” of mine that this reversal was due to Reddit becoming mainstream, and that it’s initial nerdy/techie user base preferred cats to dogs.
I think my theory is further supported by Lemmy. It’s pretty clear that most of Lemmy’s user base are techies, and currently it’s dominated by cat content. Just a shower thought. May be wrong.
NOTE: I love dogs and cats. This post isn’t arguing favoritism.
I posted a theory that dog owners are out and about doing things with their dogs and have less time and fewer hands free to make and post photos but I like yours better.