I'm pretty sure the main reason is aesthetics. And to be clear, I only saw fridges and stoves/ranges, no washing machines or dishwashers.
There are actually quite a lot of very nice smaller MCM homes in the area, Long Beach specifically has some great ones, and they are becoming fairly popular to renovate while keeping the original design. Some are quite small, by modern standards, and I think maybe the smaller size of the appliances are also attractive, but I haven't been appliance shopping much and I've never done a kitchen renovation so don't take this as gospel.
Edit: messed up formatting
I just think that if someone has the money then a company like AGA makes cookers which I'm sure have looked exactly the same for the past 60 years - so you get the vintage look but modern efficiency:
On another note, I can definitely see that modern appliances like dishwashers or washing machines/dryers would be much better and more effective than older versions. I do wonder if there's any noticable difference in comparing something like an old cast iron gas stove to a modern gas stove?
But in general, a gas stove is a ring of flame. There's not much to it. You need enough flame and you need to be able to control it. If you have those two things, an old stovetop is fine.
Ovens are more complicated and I'm not generally a fan of gas ovens.
Which is unfortunate - they are awesome looking and very functional units; there is nothing on the market like them today.
And if it hasn't been restored yet, just convert as part of the refurbishment process.
Apart from the looks, why the hell would you want 60s kitchen appliances? Clunky, loud as hell, inefficient washing machines and dishwashers are something that people not only actively search for, but are willing to pay premium for?