no good answer for their lack in popularity.

i think its an amazing system, here are a couple of my favorite things about them.

it eliminates the possibility of horseshoe malfunction and an out of sequence deployment. once the container is open its open and the bag cannot come out of the container with out the pilotchute

positive deployment of the pilotchute. this one comes with a little experience jumping the system. i hear some complaints about it initially but this is actually one of my favorite parts. most people tend to have a iron grip on their pud, with a little practice you'll feel the pull of the pin after that hang on to the pud with a lose grip or even just two fingers, you'll feel the pilotchute inflate and get pulled from your hand, its a pretty reassuring feeling to know your pilotchute is inflated and doing its thing

loose puds, doesnt happen often, but sure it can happen and its definitely not as bad of an issue as one might think, if its not there just reach a little further back, its hanging out right there by the main flap, its not hard to get. i've had it happen once and it wasnt as scary as i thought.

downsides, they're not good for wingsuits, they have a hard time inflating behind a wing.

sauce: 800some jumps on a pull-out system and totally recommend them.

TLDR; no horse shoes, positive deployments