To charge a person for a felony, at least in this country, the charge must be based on a presumption that the case to answer is beyond reasonable doubt. I confess that I don't know the exact details why Comey didn't press charges, but that absence of knowing doesn't mean that its a conspiracy. You say its an "abundant case of evidence", but the director of the Federal Bureau of Investigation formed the view that there was not enough to lay charges. I trust the man's experience and that someone in his position and office is beyond reproach.
And, frankly, if Nixon can be impeached for illegal activities, as he should have been, then Clinton as a candidate stands no chance of ducking a criminal charge if there was meat on the bone.
The standard to charge someone is probable cause, which is a lesser standard.
Neither the FBI nor its director is supposed to decide who gets charged. If there's probable cause (and Comey's speech about what the agency found was practically a brief that there was probable cause), the Bureau is supposed to refer the matter to Department of Justice for empanelment of a grand jury. That didn't happen here. Instead, the Attorney General (Comey's boss) had a secret meeting with Bill Clinton, after which, the AG announced that Comey would make a recommendation.
In his subsequent speech, Comey ignored the mens rea of the statute and imposed an intent as an element of the offense that does not exist under the law. An intent requirement that has not been required to successfully prosecute both rank and file goverment employees and high ranking ones (most famously CIA Director Gen. Petraeus). Comey further alleged that no prosecutor would have pursued charges against Clinton, despite the fact (according to subsequent reports) both field agents and field prosecutors felt she should be charged.
There's more evidence that Comey, at the very least, bowed to political pressure and a desire to not "decide the election" for the voters by recommending indictment. But that's all I recall off the top of my head.
Your comparison to Nixon is not a particularly apt one. Politics in this country were very different back in the early 70s (ie, over 40 years). In the intervening years, for whatever reason, things have gotten more partisan and members of congress tend to treat the president of their party as their party leader (basically a parliamentarian style of thinking) and one would likely never get sufficient votes from the president's own party to convict in the Senate. I would also note that the House and Senate were, in fact, both Democrat controlled at the time Nixon was facing impeachment. You also had a media that was not only anti-Nixon, but was broadcasting the congressional hearings all day on all three networks. So the public was more engaged now than they were back then.
Finally, and I am frankly surprised I have to point this out to you, I don't think you have to be of a particular political view to understand that the extremely rich and extremely powerful have been known to get special treatment by law enforcement officers and the courts...regardless of how much "meat" happens to be "on the bone.
