YoMama said:
Psychic can be used without a STAB. People like Thunderbolt so much. Like Psychic, Thunderbolt only good against 2 types, which are Water and Flying. People say that Psychic isn't good against enough types, while the only way Thunderbolt is better is that its base power is 5 more.
Thunderbolt is prefered over Psychic because when considering both without STAB, Thunderbolt has a greater base power. And yes, that 5 more base power DOES count. I mean, look at a STABed Salamence HP Flying compared to its Earthquake. Thunderbolt also completes that lovely boltbeam combo, and since Water and Flyimg types are so common in the Metagame, it makes it a great move.
Then there's Psychic, of which when used with STAB, Fighting types are unlikely to stay in (save Heracross and Medicham in some cases). Add that to the fact that there's almost always a person in your Party that can earthquake better than BLissey or someone can use an UnSTABed Psychic, and Thunderbolt is ultimately the better choice for an unSTABed special attack.
And to make sure I've clarified my point, look at the super effective capabilities:
Psychic - Poison/Fighting
Thunderbolt - Water/Flying
Once again, i say that water and flying types are THE most comon types in the game, where as Poison and Fighting types aren't to be as feared as much. A team should be balanced enough so that switching will complete a defensive capability to handle either of the two types anyway.