It isn't random. EE is the only storyline (so far) with substantial crunch of its own (the EE spells and races).
The point is to prevent people from mixing and matching the best from two separate sources of crunch. This way, the risk of inadvertently allowing a broken combo is minimized.
In effect, you're meant to take the PHB plus one (1) supplement and build your character from there.
This is why you can't be a Genasi Purple Dragon. It's not random, it's because that character would draw from more than one supplement: both EE Player's Companion and SCAG.