Why not use monochrome LCDs instead of E-ink?

Check your calculator: the blacks are good, but the whites are very bad. This happens because the LCD is good at blocking light, not very good at letting it through. You don't notice this in normal LCD screens because the manufacturer can just ramp up the backlight until the whites are acceptable. Since eReaders are meant to imitate paper, bad whites are a big problem.

If you think about how LCDs produce images, this makes a lot of sense: light always passes through a polarizing filter that absorbs a lot of light, leaving only the "correctly" polarized part. The result is less light being reflected.

Thanks to a different recent question, I found that it has been tried and is called "memory LCD" and is used in the Pebble smartwatch.

The key is that normally LCDs are driven by a pulse every refresh period, and will fade if voltage is not maintained across them. Sharp have addressed this by adding to the conventional thin film transistors a memory element of some sort. Their power consumption for static images is very small, although not zero.

I think the LCDs are still necessarily glass, while e-ink displays can be slightly more flexible. This is a fragility disadvantage.