"what I don't understand is...why is everyone buying the exact same aircraft. "
Perhaps because they are not?
The variants have diverged so much from each other that apparently there is now about 20% commonality between them.
Which (probably) will cut down the logistics costs compared with supporting 3 wholly separate designs, if you operate multiple variants.
Will it be enough to justify the development budget to date?
Who can say?
Biting the hand that feeds IT
