I hope the wizards will forgive a down-to-earth analogy.
Either a b is a knapsack with two pockets, the one on the Left (which must hold an "a") and the one on the Right (which must hold a "b"). But you can only use one pocket at a time.
So when there's a specific case of Either a b such as Left True, all that can be concluded is that this is a case of an Either whose left pocket must be able to handle a Bool; there's not enough information to know what could have been put in the Right pocket. So only the left-pocket type ("a") can be replaced with something specific.
Similarly, for a specific case of Right False, it's clear that the right pocket holds a value of type Bool (replacing the general "b"), but there's no information to identify what type might be in the left pocket. So it remains unspecified.
I hope that helps.
-jn-