Nixon was almost impeached for sticking up for his cronies and withholding evidence (high crimes and misdemeanors). Clinton was impeached for perjury and obstruction of justice. Johnson was impeached for high crimes and misdemeanors. Why can’t Obama be impeached for some of those very crimes? (Plus, at least half a dozen others.) the answer is because the Republicans won’t do it. Not even for the sake of Principle. They are nothing but oxygen thieves at this point.
He can be.
Your question should be, "Why hasn't Obama been impeached for his many high crimes and misdemeanors?"