Well, there are two problems with your 100 base power argument:
A: this isn't Pokemon, Rock Smash would have no attack purposes whatsoever otherwise
B: It may not be earthquake. The name of the actual move is unknown, but it looks similar to Metagross' attack, which is known (Earthquake)
Also, Dsmash is a punisher, which aren't as recommended as other moves. Here is kinda the reason some attacks are rated high, and low:
Practicality: Compare Fthrow to Fsmash on this one. Fthrow happens quickly, your opponent pretty much has to suffer it, and it can easily link to combos. Fsmash, on the other hand, is extremely slow and hard to hit with, especially charged, and is ranked low.
Range: Charizard has only one move which I say "don't use it" and it's for a good reason, Charizard has some pretty large and disjointed hit boxes. Flamethrower, Rock Smash, the grab, and most of his aerials have a rather large area it affects. The fact that Dsmash only hits grounded opponents horribly limits this.
Spamability: Ever wounder why none of Charizard's specials are 5? This factor, how good is it at spamming? With his specials, they are all HORRIBLE at it. Rock smash executes too slow, Fly has a weird hit box effect, and flamethrower has an anti-spam mechanic. Grabs can easily be linked, and Bair is rather good at this thing. Rock smash, not so much.
Overall hurt: Now, there is two types of hurt, instant hurt and overall hurt. Instant hurt means how much an attack actually does at that instant (Rock Smash is incredibly high here). Once past that instant of pain, do you have the ability to link it to something else? Overall hurt means how much pain you can potentially deliver AFTER and attack. Grabs can easily be chained together, adding to the overall hurt. So far, we have found no combos with Rocksmash, so while the instant hurt is high, the overall hurt is rather low...
OK, how is that?