Ricimer was a Seuvic general who essentially was the de facto ruler of Rome in it's last few decades of existence. During his time, he set up and removed four Emperors in the Western Empire. Why did he never declare himself Emperor, if he was in all but name so?
I'll try and answer until someone more qualified gets here.
People wouldn't have it. Ricimer was a German, with no right to the purple. The Romans had proved time and time again that they were alright with a wide variety of people holding the power behind the throne. If we go back to Tiberius, his mother held considerable power, if she wasn't indeed de facto ruler.
However, could she be empress? The people would have laughed, the Senate would have been blown back, and the army would put on their civil war boots. It's the same with Sejanus. He was basically the ruler at a different time in, again, Tiberius's reign, but when he requested marriage to an imperial relative, he was denied. His descendents would rule the empire in that case, and Sejanus was an equite of all things. The horror!
Women couldn't rule in their own right, and until Macrinus, neither could anyone but those of senatorial rank (and they killed him before he could even step foot in Italy). A German, a German? Those northern savages? What, is the empire a tribe now? I'm not going to be wearing any blood soaked wolf cloak or paint myself blue or whatever it is they do anytime soon, thank you very much.
People accept power behind the throne, as long as it stays behind the throne. This is more conjecture now, but I think that we're alright with that because it's a lot harder to get rid of someone once they get a title like Augustus. A bad advisor is just a man. The Emperor is Father of the People, Imperator.
It's important for images sake that a Roman wears the purple. They can handle being ordered around by most anyone, but being Imperator is something special, more than just the guy at the top. That's something very human, and I believe Ricimer was wise not to step on that.