First of all, forgive me if someone else has already posted something like this, but I don't have time to be scouring the /r/minecraft archives for a post I don't know exists. Looked through the first two pages, so I hope that's good enough. Also, wall of text, so sorry about that.
I have been seeing some comments from users here that accuse Mojang of being lazy for not implementing Mobs A, B, and D. Those that say this miss the point of the entire exercise (or, at least what I see as the point).
Mojang dignified us as the community by giving us a choice on something to be added to the game. They allowed us to influence their development process. And while this is nothing new, I think many of us, myself included, forget just how much power they give us over them.
I'm going to use an example that comes to mind, but I'm sure you can think of many, many others if you try. Months ago, Mojang added a potion effect called Luck. This was tied to a mechanic in the game that never materialized, and the potion effect was removed. Then, there was an outcry. So, Jeb put it back in.
Why, and what effects this had are irrelevant at this time. The point is, they listened. This team, who could very easily abandon its core audience and start marketing solely to impressionable young children, is still listening. They still care about what we think, even though they could go right on and ignore us. When they gave us a choice of mobs, they are carrying on this pattern.
So, what if they had simply implemented all four mobs? What happens to that choice? They'd be pulling out from underneath us. The choice would have meant nothing. It would be like when you put a child in a car with a steering wheel that does nothing. It offers no control. It is a farce. We, who are older, can see what the child cannot.
So, while maybe they could have handled the matter better (who of us handles everything we do perfectly?), they did the right thing. They made sure that choice mattered. And it did.
Also, in the words of Benjamin Sisko, "You cannot destroy an idea." The idea for those other mobs is still there, and they still have a record of it. Perhaps they will make a return at next year's Minecon, and that will allow us another crack at that enchantment eating monster, or the shielded blaze mini-boss. If they do, I'm sure that there will be some that say "Just make them all!" and don't realize the disservice they'd be doing us if they did.
I won't be complaining, though. It is, after all, our choice.