"They already have it on PC, can't they just copy it from the PC?"
This is probably the biggest question I receive in the Powered by Redstone mailbox, next to when 0.9.0 is coming out. It usually comes from younger people who visit our site. We get asked constantly, "when are all the mobs coming to Pocket Edition, can't they just copy the code from the PC?" or "when is all the biomes coming to xbox, can't they just copy and paste it from the PC?
The answer is a resounding no. No they cannot. But to someone completely unfamiliar with the basics of programming, this usually doesn't help. So let's explain why.
You probably know what programs are (they are also called applications, usually on iOS and Mac). You probably think of Chrome, or Firefox, or a game as a program. Which it is. A program is just a set of instructions that your electronic device reads and follows. Those instructions can do anything from load our website on your phone to displaying a game.
Programs are written in more than one language, and a program can even use multiple languages at once. Popular programming languages include C++, C#, Java, C, Objective-C, and more. Every edition of Minecraft is written in a different language, to take advantage of whatever platform it's running on.
Minecraft PC is written in Java, which even though is extremely resource inefficient and not exactly designed to power video games, Mojang has decided to not change it because it would make modding more difficult (and also because it would take a very long time).
Minecraft Pocket Edition is written in C++, the console editions of Minecraft are written in C#. So as you can see, every Minecraft edition is written in a different programming language.
So we come back to our original point, why can't Mojang just copy features from one platform to another? Because in addition to converting it to another programming language, most of it has to be re-written anyways. For example, dimensions can't be copied from PC to Pocket Edition because Pocket Edition handles world files and multiplayer differently.
There are so many other examples, but you get the point. So no, Mojang can't just copy and paste stuff from the PC Edition to other platforms. Although many parts can certainly be re-used (like textures, and some other elements), most of it has to be re-written for each edition of Minecraft.