Answered on the download page - each camera port is done by volunteers who own the camera, in their spare time
. There's no Magic Lantern organization that does project management or gives priority to this or that camera model - it's simply the camera owners showing enough interest for implementing or porting these features (not by asking, but by updating the source code on their own initiative).
So, your question could be rephrased as "why are the 5D3 and 700D owners reverse engineering their cameras, and publishing their progress without waiting for 70D owners to catch up?!"
FYI, on 70D there are some important unresolved issues (FPS override, raw overlays in photo mode) and so far nobody found the answer to them.
Also FYI, most of the recent progress I've been posting lately
applies to 70D and all other cameras. However, a 70D owner
(I'm not one of them) has to follow the same steps and customize them for each camera (rather than complaining) - and the same is true for most other camera models.