Quora: Why doesn't Donald Trump just appoint a special counsel to investigate the Bidens?

Bruce Spielbauer answered.

Because, Trump would have to state the reason (some tiny bit of evidence) — and there is none.
It is the same reason Trump did not just call up the Department of Justice, and ask them to investigate. Either would have been legal. (If Trump had a tiny bit of evidence.)

Asking a foreign power to do it, (both the Ukraine, and now China) was a crime. A felony, in fact.
Because, the subject was a political opponent.