Because many people prefer reading on an eBook reader over a tablet:

- Cheaper ($149 or $249 compared to $499)

- Easier to read (eInk is a lot easier on the eye when reading for long periods than a backlit display)

- Not confusing (Granted, an iPad is pretty easy to figure out, but for an average consumer a device that does one thing is generally easier to grasp)

- Better battery life (Nook claims 10 days, iPad claims 10 hours)

The two technologies may converge in the future, but for now, the dedicated eBook Reader still has its place.