Diuretics, amphetamines, and other means of performance enhancement and body alteration have been used besides steroids. These are all banned substances under the UFC's rules. However, none have been as vilified as steroids. This makes doing them very shady business for fighters who are weary of image, marketability, credibility, flexibility, and the bottom line of their contracts.
For fighters who are weary of their image, steroids can be a death knell. Hermes Franca was a fan favorite with wild powerful punching and dazzling submission skills. Fans enjoyed every trip he made to the octagon.
However, this seemed to change after he tested positive for steroids at the Sean Sherk fight. He was suspended for a year and his purse was partially stripped.
A year ban from a major promotion is a bad thing. For one, an owner or president as judgmental as say Dana White, may not have you back at all. That is grounds to terminate a contract. Also, fans will never fully trust you again as they did prior. This is a major setback.
As for marketability, take a look at Sean Sherk. For Sherk life was good, he was the muscle shark, the rough, tough guy who could dominate anyone. However, when you follow Sherk's career he was a relatively unsuccessful 170 for some time. How the sudden bulking and weight dropping? Doping. Sherk finally got caught with his hand in the cookie jar and has not seemed himself since his return and loss to BJ Penn.
Credibility is also damaged irreparably. Royce Gracie tested positive for steroids after a fight that he had in California, now some speculate whether his legend is as great as it seems, or if he went to Japan only to enjoy lenient drug policy. These have all made Royce become an increasingly shady role model.
Flexibility is always a major part of MMA, whether doing submissions, throwing high kicks, etc. So why would steroids be a good idea? They're not. How can you use your increased strength to lock a triangle when your joints are stiff and immovable, how can you land a stronger kick when you can only throw body kicks. You become one dimensional. Much like Bob Sapp has become. Sure he can punch, so work around that and you win.
Finally, the bottomline on your contract is damaged, much like Phil Baroni's was. Phil tested positive for steroids in EliteXC and was probably a long time user. Not only did it make him lose a lot of money of his purse, but also put him out for a year. That's three or four fights.
As a note I would like to add that the UFC and other corporations such as Strikeforce and the WEC have implemented much stricter drug policies to deter drug abuse. Let's hope we can stop this problem before MMA ends up going to congress.
Although steroids may not be buried with Sean Sherk and his shrunken testicles retiring from the UFC, it will certainly not be as common as it once was.