tl;dr - It's like a crawler (mobile with a boom, street level crane) but you put it on top of something already there. Since its on top of something, it's not a "mobile" crane and couldn't get certified as one, however, it also didn't fit into the tower crane (tall, lifts things) certification. So it got stuck in between two classifications and as a result made it too hard for construction companies to bother includi…
Then it was unapproved, thanks to political pressure.