Cost: one speaker and tubes was probably cheaper than 200.

Weight: old headphones were chunky, see above.

Comfort: not wearing giant chunky headphones you weren't accustomed to might have been preferable

Breakability: pneumatic headphones were harder to break, cheaper to replace

Stealability: passengers would have no reason to steal the headphones, and if they did it was cheap to replace.

All of these go away as proper headphones get increasingly small and cheap.

I'm pretty sure there were 2 speakers in each arm rest. I think it's more about them being harder to break and cheaper to replace. When I flew on an airline that used them, they came clean in a sealed bag. They may have been brand new, or at least cleaned and sterilised. Electronic headphones are unlikely to be as cleanable.