Barr did not control the secret hearings in the House. They had complete freedom to call whomever they wanted, and did. They probably knew Bolton had nothing to offer. “Sufficient evidence” was not needed because the House majority could impeach a president for saying “howdy” instead of “how do you do”. They don’t need “probable cause”, and didn’t have it. They didn’t have a single witness, not one, who could testify that they ever heard Trump make a quid quo pro demand or ever say he did.
The Repubs in the Senate took the role of jury and acquitted. The Democrats just didn’t have evidence to convict. They could do it again, and probably will right before the election. It’s totally political.