Because you develop for the PC then port to the consoles.
Not quite. The PC version is an Alpha build that is still having core features developed. The upcoming release of a17 for the PC isn't just a usual update with a few features either. Almost the entire code has been re-done from the ground up. Not just because of upgrading to Unity 2017/18, but also because entire systems like buffs and zombie AI have been re-written from scratch.
The console version on the other hand, was taken as a finished product on Alpha build a14.7. It was pretty stable, but still had a few bugs. They ported that to the console, and have been getting updates from The Fun Pimps as items are deemed stable/complete enough to be ported over. If a system is unfinished (like electricity has been), it will not likely be ported over to the console. (Which is why I'm always amazed that they put the minibike in the console.)
The console version is published by TellTaleGames, and is being developed by Iron Galaxy. This is completely separate and independent of the PC Alpha development. So much so, that you can almost consider them two wholly separate games.
Now the Console version is being updated to the newer version of Unity. I imagine you will see things like painting, the crossbow, updated zombie types, and possibly electricity. I do not imagine that you will see things come to the console like the new cars, the new buff system, the new AI, or bandits and survivors.
Which isn't to say the console isn't getting those things. Hell, if TFP things they have it stable enough, and IG is able to make it work within the horrid system specs of the console, it just might be in the next update.
I wouldn't get your hopes on anything huge though. I imagine you'll get the rest of what was in a15, a good part of what is currently in a16, and a very good bit of optimization. Which may be able to lead to expanded worlds, and a better multiplayer experience. Hopefully they'll also have found a good way to work around the bugs caused by Microsoft's horrible file system.