I’m not sure what you’re reffering to when you say “HHKB”. The layout? Topre? Both?

If it’s about the layout, HHKB is a “perfected” 60% layout for people that enjoy UNIX layout.
The R4 Control keycap is missing as there is no use for it, letting space for deliberate presses on Alt/Super key, and same goes for the right blocker, the open space there makes so that you never miss your Fn key (a very important one in this layout).
The beauty of a 60%, and HHKB implicitly, is the fact that you do not have to move your hand too far off the home row when using the keyboard. People prefer using layers, split 1.5u Backspace and UNIX Control for that exact reason.

As for Topre…that’s just preference. The switches aren’t like anything else on the market. They have a much more “meaty” tactility compare to the “toothy” tactility MX offers, the silenced version sounds very clean with no weird metalic noises from the leaf or spring, and the bottom out is much more firm than regular robber domes over membrane as you have a solid PCB under the dome. Also, compared to membrane rubber domes, you don’t need to bottom out to activate the switch.