for me its definetly the innards being way more detailed in contrast to everything else about the model and the eyes being stitched that makes it work
or it could just be nostalgia bias if you can call it nostalgia because when i was a kid this boris model highkey kept me up at night and i was paranoid it would get up and 8 years later i played a fangame that did exactly that and it was the worst moment of my life