Possibly due to the many articulations on Figuarts requiring a lot of work for posability, while these figures have more restricted articulation. Not that the articulation joints are any good on the bootlegs anyway, most of them snap right out of the socket inside the box!
But to be fair, that also happens to official figures. I just received my SDCC Prototype Ronin Boba Fett and right out of the box the leg snapped off the ball-joint and then the torso snapped off the waist joint, but thankfully there was no breakage so I was able to pop it back in. Official figures have far better joints and articulation, but it's still not quite perfect on some figures, and seems to particularly be the case on the exclusives, I remember the same thing happened on my SDCC Sandtrooper Corporal.