Hi Chani,
My best guess is, (and my personal idea bout) that TV standards (UHD) and its processing, as well storage option will be a reason.
Since over ten years we see that computer and TV-sets merges more and more. This huge shift will take time, based on the hardware installed in hundred of million households. Typically not the best solution wins (Beta vs VHS for example), it might win what reaches into the most households, or what is the most cost effective. Sadly, not often is what technically possible as well the final decision for the end user. Production wise, that is a different story, there we have to keep it hi-end. Perhaps, this time we can see a change, since smart-phones and tablets seemed to have a typical one year cycle/lifetime these days, and represent a mix of TV and computer. ;o)
I shoot since decades panoramas, and in case of 360ºx180º my typical storage is the equirectangular projection. Is it the best option (?), not in general, but for mainly horizontal oriented users, it might be. The Zenith and Nadir areas are heavily distorted, as even a quick look at it unveils easily.
If we talk about “stereo” or 3D-VR, then the format would be, based on eqiurectangular, better 4:1 or even 1:1. Or oscillate the channels…?
Going back to my research about VR in the early ‘90s, the main vision was, besides HMD [head mounted displays] “The Cube”, a six sided box with large 1:1 screens. The “Cross” available as conversion in C4D, would be the better solution, given less distortion, and a better workability during production, fixing patches, etc. This format is 4:3 (or 3:4) and would have been better suited for SD back in the days. Things changes. Perhaps we will see a six panel view packed closer than the cross for this, which would be 3:2, close to 16:9—again.
For now, my best idea is, to produce always above the needs, and sample down. I do that with my RED in 6K (funnily enough a 2:1 sensor, well: 6144 (h) x 3160 (v)) or with my photography, for 3D in 40K+ 32bit/channel/float or for my art in 16K+ at least 16bit/channel/integer equirectangulars. Not in sRGB or REC 709, since we move to UHD and REC 2020, with HDR options.
Bottom line for me, I would go with YouTube and 16:9, but if the target market is FB, well, then we know already where it goes, given the bad image quality they allow now. Besides that, Google likes to produce a representation of the world, FB does not so far [except for twenty cities, but many years behind Google]. It is more likely that Google owning YouTube will push this very hard. Yes, FB declared this to be a tool, but between want to have it as tool or that the mainstream use it is a long way. In all fairness, google “glasses” didn’t make it.
My prediction is, as long as the GUI is not changed from desktop to “office space”, VR will grow slow, except for games.
As a vivid “Making Of” fan and collector of such, I can already see that this will be a feature on Blu-ray™ discs especially with the upcoming UHD discs: Behind the scenes or behind the stage documentaries. Which would certainly work nice if available from the start in UHD, which is: 16:9. But well, things have often played out differently, based on strong none technical influences.
Again, this is my personal view, I guess the developers of the plug-in have other sources to share or opinions.