That's not just about capacity, it's about price too!
If one can even consider the $250 asking price of the diminutive Dan A4 case (when other widely-available SFF cases on the market can be had for as little as $50), I honestly don't think they would be too worried about the cost of storage.
I personally don't think it's fair for the designer of this niche case (or any sub 10L case) to invest in the tooling costs necessary to accommodate for extremely large budget components that many people in this particular enthusiast market would consider "not worthy" in a SFF case.
Regardless of cost, a 3.5" mechanical HDD consumes .377 liters (that's 1/10th of a gallon!). That can make or break a crowdfunding campaign when a vast majority of their target audience wants nothing to do with mechanical drives nowadays.
Thankfully, there are already a plethora of cases available with support for 3.5" drives. For everyone else in this small audience of computer enthusiasts, our choices are somewhat limited.