Because the Fernspaher have better accuracy, lower rate of fire and 5(+1) men, while the 4VPZU have 10(+1)?
Fernspaher are 10 man squads, and yes they have better accuracy but slightly lower rate of fire. I only use them in Fuchs (In the same way that I only use Spetz in the BTR)
You obviously don't know much about them, so I recommend that you try them considering they are one of NATO's best infantry units.
Oh, my bad, I mistook them for an other unit (BGS, they definitely suck). I know these guys, but their horribad air transport (the ground one is very good) and lack of LAW make me frown upon them, as there are units with similar anti-infantry abilities but better versatility (SAS with their strong flashbangs are more efficient against infantry for the same price, but no land transport, while Deltas prove monster anti-tanks and monster anti-infantry, again only airborne and with slightly worse optics.
BTW, I'm the OP... So I know what I'm talking about (screwing up units names happens).
"Rocks are OP. Paper is fine" signed: Scissors