I don't want NASA messing with the latest technology just for shits and giggles. Don't fix what ain't broken.
I can watch much higher quality videos streamed over the Internet, from servers thousands of miles away, around the world, through many routers, fiber optics, and complex technological stacks (DOCSIS, TCP/IP, fiber optics signal modulation, video codecs, etc).
Yet the ISS has a clear line of sight to ground receiving stations 230-1000 miles away (or to satellites which are perfectly able to transmit digital data), yet the video signal quality is that bad?
Don't get me wrong. As a space enthusiast I love being able to watch live spacewalks. But as an engineer I sigh at certain technical aspects...
What would be the point of a higher quality video in this context? Is it necessary, or simply a nice-to-have?
One of the most popular axioms of engineering is "if it ain't broke, don't fix it." It could be revised for this scenario: "if it gets the job done, don't improve it."
Unless you mean that digital links aren't more vulnerable to radiation, in which case I recommend you read up on large-scale analog circuitry and get back to me.
Wait for SpaceX or somebody else to bring the per-kg cost down and we'll have all the digital video anyone could ever want.
I doubt it.
Like someone said, initially they chose analog because back in the day nobody did digital video streaming. It has worked okay since then. And there just hasn't been enough reasons to justify a change to digital.
Also, for a spacewalk the video source is from a space suit, so the video signal is sent wirelessly from the spacesuit to ISS, then sent to a TDRS satellite, then to the ground. The spacesuit camera is admittedly not as good as I'd like it to be, but considering all the architecture supporting it started being put in orbit over 10 years ago, and was designed before that, it's pretty good.
Crazy stuff.