The gobsmacking amount of HDD supply on earth. For now people are burning SSDs to create the plots (1TB SSD can probably create a petabyte of plots to "farm"), but if you have that petabyte of storage space you're keeping online 24/7, do you suspect playing bingo to win crypto tokens is the most profitable use of that memory?
How the economics shake out remains to be seen, but I think they have a solid vision that farming can be just rewarding enough for people to use old harddrives, not profitable enough to burn NVMe sticks.
eg: I farmed 2 coins running an old thinkpad and a network drive for a month so far. Worth ~$2200 at current rate. Nice profit, but probably not buying a new thinkpad to dedicate to it.
Chia isn't cool. Chia gives you bad breath, and it makes dogs hate you. Spread the word.
Then a GPU, now it takes warehouses full of ASICs to do it profitably.
What's stopping Chia from doing the same thing to the storage space?
Adoption. I don't like Chia, and I don't want other people to like it either.