Why isn't he anywhere near as strong as the Thing?

ive never even heard of rock slide but its kinda like asking why is Professor X a stronger telepath than anyone else. the same power varies between ppl would be my first guess

The Thing is the F4's strength guy.He's an original Marvel Titan.The X-men have Colossus.

Thing is not made from Rock. He has a hard orange skin that resembles rock. So they are not made from the same stuff either.

Thing does not also crumble when things get really tough

Yeah, that's my thought. Rockslide crumbles too easily. Enough force and he breaks apart, luckily he can pull himself back together. Maybe he just needs to break up around stronger rock to pull himself together again. So basically Thing is rocklike but much stronger than any rock on Earth while Rockslide is made up of earth rock.

I figure its because thing has muscle under his rock-like exterior while Rockslide is all rock. Also thing is tougher.

Rockslide's just a teenager. He'll likely get stronger.

Rockslide isn't the same as the Thing.  Rockslide is a psionic being who constructs a physical body out of stone.  Thing is a physical being made of orange rock-like substance.  Similar, but not the same.  Also, Thing did have years of wrestling experience before Rockslide was even born ;) 
 
But this would be a fun fight to see, if Marvel ever thought of moving their mutants off Utopia ever again :p

Rockslide lacks the musculature underneath the rock to give him thing level strength. 

Wow, as a person made of rock, I am offended you just assume that all rock people are strong. Do you assume all white people really lack the ability to jump? That all Asian people know kung fu? That all old people are senior citizens? Why not ask why Anole isn't as strong as Thing huh? Oh wait, he is green, so why isn't he as strong as the Hulk eh? 

It's because the Thing has Brooklyn rage.

" Wow, as a person made of rock, I am offended you just assume that all rock people are strong. Do you assume all white people really lack the ability to jump? That all Asian people know kung fu? That all old people are senior citizens? Why not ask why Anole isn't as strong as Thing huh? Oh wait, he is green, so why isn't he as strong as the Hulk eh?  "

Strength is not always the determining factor in a superhero battle. What counts as much if not even more is battle experience and fighting spirit(what used to be termed "moxie" by Americans), therefore the Thing or even Colossus would/could defeat Rockslide; they after all have been around the block more than he has!

Terry