The High Definition footage for This Is It's short films were shot in 4K using a Red One M camera and you can see Michael using one below. The camera is capable of shooting in a higher rate rate at lower resolutions (50-60fps at 3K (windowed) and 75-120fps 2K (windowed)) however I've yet to see anything that suggested these were shot in a higher frame rate, so I'm going to assume 4K.
For the actual rehearsals, like MJBT says below, they were shot in 1080p. Not entirely sure what camera they shot the standard definition footage on but you have to keep in mind that they weren't expecting for this to be seen by the world, let alone shown in theatres. We can really only guess why they shot some of the footage in standard definition as none of the crew has ever spoken about it.
My guess would've been probably the cost of it all. They wouldn't have shot everything with Red cameras as they can
easily run into the tens of thousands of dollars (I think around $40,000?) and then you have all this footage which would take up astronomical amounts of space. A single 4K frame from an EPIC camera in the R3D file format is approximately 1.5MB. Assuming they shot in 30FPS, that's 45MB a second. 2.7GB a minute. 162GB an hour. You get the idea. Multiply this by the amount of hours every single day by all the Red cameras they used and it adds up fast. This is all assuming that through post-production, they stayed in the native R3D format. If they converted it to another format, which I've read isn't uncommon in Hollywood, it could easily be
even higher. Considering they likely used Sony HVR-5ZE's as MJBT states below, the file sizes probably weren't as high, but were still quite higher than those for the standard definition footage. Once again, seeing as this footage was never intended to be viewed by the public, they probably didn't see the point in justifying the cost.
Standard definition cameras on the other hand were much cheaper. The same amount of hard drive space that could only contain so much HD footage was able to contain soooo much more SD footage. It's also not necessarily just the cost of storing all of this, there was also media transfer time - aka the backing up of footage. As HD footage tends to have a lot more information and therefore take up a lot more space, it'll take longer to back up from the camera to a hard drive and time is money. Someone has to be paid to transfer all this footage. In addition, as this was entirely digital, they have to back up the footage to new drives every few years (which is why even films shot entirely digital nowadays have their final cuts are printed back onto film and archived in that way as well).
There's a reason why you see security cameras shooting in standard definition even in today's world. All that footage has to be backed up somewhere and to store a lot of footage can take up a lot of space and thus money. If it's shot in SD, more footage can be stored on the same hard drive and you therefore have a wider time frame of footage to view when you looked at the security recordings. Same concept could apply here, they have a wider timeframe of Michael's performances for cheaper.
That would be my guessing.
