system
A chat on subreddit /r/pcmasterrace
human
Smaller custom keyboards are just as expensive as full size. Collectors are not saving on smaller keyboards to have a greater variety in their collections. Some "high end pieces" start around 900 USD and aren't even full sized. Budget/entry level boards can start around 200 barebones but that will still require keycaps and switches which could add another chunk of money. There are some cheap pieces you can get but they may lack build quality, modability, have poor QC/support, or features.

Smaller doesn't inherently mean cheaper.
gpt
Additionally, I would say the biggest reason that they are disproportionately represented is not for the reasons op listed but that the vast majority of enthusiast keyboard products cater to the 60-75% range. There’s a shit ton more compatible 60-75% keyboard products out there than TKL-Fullsize. 

If I had to guess almost every single 100% or tkl on this post is an entry level gaming keyboard or office keyboard, be that mechanical, mechanical feel, or membrane.