Why are derailleur jockey wheels commonly made of nylon?
Caveat: I've only seen two or three sets of metal pulleys in my life. There are several reasons:


cost: plastic parts are cheaper to make than metal ones
weight: it's easier to make a lightweight plastic part than a metal one (this is also related to cost, "easier" usually means "cheaper")
durability/resilience: somewhat counter-intuitively, plastic cogs are less likely to fail completely. When the plain bearing locks up the plastic wheel is likely to slip on the metal bearing (and many people don't even notice that this has happened), or the teeth with abrade off to form a smooth surface that the chain will run on (it's very hard not to notice this). A metal pulley does not do this, if they stick for some reason generally the bike isn't rideable, and it's unlikely that the aluminium pulley will slip nicely on the steel bearing.


The latter is somewhat hard to get experience with, since metal pulleys are sufficiently rare and expensive that few people use them. Also, people who pay extra for them tend to maintain them so I haven't seen them fail other than via "full of mud, not going round". In that case, though, the chain will not go through the rear derailleur at all, so the bike needs to be converted to single speed before you can ride it. Which is an ugly thing to have to do when you're out on a bike ride.

Note that recumbent idler pulleys have the same concerns but often they're on the drive side of the chain so the forces are larger. So are the pulleys to reduce chain bend angles and thus losses. In that case metal cogs are usually also lighter and quieter but significantly more expensive (more than double the cost). Derailleur pulleys would also benefit from being larger, but that's relatively rare, I suspect because weight and air resistance also increase with size (and a larger cog would mean extra chain, further increasing weight)