They didn't take Blu-ray because it wasn't slot load and it didn't have a good battery life.

And, because blu-ray sucks in terms of draconian licensing issues that require manufacturers to limit the capabilities of the operating system AND hardware in perpetuity.

Apple doesn't really like to be told what to do by others in the industry.

Apple will go with Blu-Ray only once Blu-Ray doesn't require unilateral, legally-bound platform restrictions by the licensors.