Earthquake actually was a sumo wrestler in Japan.
I think that you're asking why we don't see big fat wrestlers often anymore rather than why those wrestlers died, but correct me if I'm wrong. The expectations of the typical fan today are different than they used to be, particularly when it comes to the pace of a match. Guys like Mabel would never be able to keep up today. It's easy to imagine an athletic guy like Earthquake keeping up, but he'd likely be thinner. By the same token I could see someone like Walter being way fatter if he were wrestling 25+ years ago.
problem too is Vince doesn't book them as legit threats either. In Vince's world if you're fat you're a comedy act jobber.
Really, outside of Yokozuna and Kevin Owens (and maybe Kamala) any fat guy that got over in the WWF/WWE was an accident.
Rikishi - was a comedy throw away with Too Cool (who themselves were meant to be jokes). but they got over huge with the crowd and forced Vince's hand
Dusty - he put him in the polka dots and had a big old black woman manage him (she was awesome btw)
Foley - got over on his own
Bam Bam - never got the push he deserved
Vader - see above
Samoa Joe - see above
Brodus Clay - got the Funkasaurus as a punishment and got it over on his own
If you're a fat guy who got over in a different promotion and come in with an established name, then you're double F'd...those are Vince's 2 biggest strikes