my guess - e ink refresh rates / ghosting suck and it would be hellish hard to get an on screen keyboard with an e ink display to get anywhere the typing speed a modern touchscreen can deliver.

The article actually mentions that:

"I really appreciate them including the keyboard here, as the display looks great but is definitely not all that responsive, so typing would be a lot more frustrating without this."

I don't know how much the fast refresh rate mode helps in that regard.