According to http://www.simetric.co.uk/si_metals.htm brass is 8400 – 8730 kg per cubic metre, stainless steel is 7480 – 8000.

So, if spokes are made of stainless steel, it’s lighter than brass and less likely to round off under the spoke key, why isn’t it used for nipples ?
Is it because stainless to brass is less likely to seize than stainless to stainless, or is there some other reason ?

Stainless is actually stain-resistant. Will still rust eventually but in regards to your question, idk…

Steampunk wheels, innit.

“self-lubricating” features in the answer but I’m not quite sure how it really matters

Stainless on stainless has a tendency to gall, especially on threads. Causes it to bind or tear up and seize.

I thought stainless was a bit softer and maybe not able to take the tension so well on fine threads?

Stainless to stainless is horrible for gallng !

A bit of internet research turns up a discussion amongst people who seem to know what they are talking about.
http://www.practicalmachinist.com/vb/general/why-does-stainless-steel-gall-225765/

It seems stainless to stainless can even gall under finger pressure alone.

used to be a running bet with the new guys at Screw Products that they couldn’t take a 3/8-16 18-8 nut and run it by hand all the way down a 36″ length of 18-8 all-thread. I don’t think anyone ever did it.

Brass is nice & soft & easy to machine, especially for rolled threads. Stainless isn’t. Plated mild steel would rust as soon as it sees a spoke key.

Odd things spoke nipples, look like they are formed/spun rather than turned.

Galling is just the grain structure locking isn’t it ?

They have to be able to turn when the spoke is tight, enhancing the friction force in both the spoke/nipple thread and the nipple/rim interface. I think brass does well on friction. Also, is it better if the nipple wears out or the rim?

Im not sure about bike use but I have found stainless to be a bit more brittle when used in screws compared to brass. The heads tend to pop off when under a bit of stress whereas brass will take a bit more punishment.

Stainless nipples would gall up as soon as you look at them, you can coat the threads to prevent this with copper but you still only get 6-12 make and breaks. Wouldn’t worry about the strength of them – 13Cr is used alot for couplings, crossovers and hanger bodies on undersea pipelines.