That's easy actually to explain why. Almost all toys today are created in 3d on the computer. So, that model is produced for animation, etc, and then after the 3d model is complete, they can migrate it to be produced at the factories. Although, that process takes time and just isn't done with a snap of the fingers. The 3d model is then digitally created (probably by 3d printing techniques, then they have to produce molds, then setup the manufacturing of it and then the ability to mass produce it.
The 3d modeling process is the first step, and once that's done, video games and animated movies can easily use that within days after it's created. It will take many months before actual real world lego prototypes are readied for production though..
I think it was a year or so before the Attack on Hoth set was put out, where we saw an animated film of the exact set. That animation turned out to show the set that was about to come out. I'm guessing this Bantha is very much in production. It will be a little more than a year since that image of the lego bantha came out from some animated clip. Regardless, I think they could make a set similar to the cantina and have all the figures in there (luke, obi, c3po, 3x tusken raiders and a maxi bantha) and charge around 69.99. They can just produce a landspeeder and a little rock cliff where the Sandpeople attacked Luke before Obi rescued him.