Because I use that monitor to read and write text. Lots and lots of text, very little video. 7 fps is abundantly adequate for reading text if you only interact with it by hitting 'page down' every 30 seconds, and if your typing is accurate enough that 150ms delay between hitting the key and seeing the character doesn't cause errors it's fine for writing text.

I don't need 30, 60, 120, or 144 Hz refresh rates to work with text. It would probably be impossible to see the difference. What I need is contrast and brightness settings to limit eye fatigue, and that's where e-ink would be great.