Comments to the previous version of the rule indicated that misconduct included knowingly demonstrating bias when that conduct was prejudicial to the administration of justice. Model Rule 8.4(g) expands that concept to apply to the practice of law more generally, rather than primarily in courtroom settings. Some question whether the previous version of the Model Rule was sufficient to address bias and lack of diversity.
First, Comments are not Rules and do not have the same authority that is found in the Rules themselves. (See Preamble & Scope  (2016) to the Model Rules of Professional Conduct: “The Comments are intended as guides to interpretation, but the text of each Rule is authoritative.”)
In reference to existing antidiscrimination language in the ABA Model Rules prior to the adoption of 8.4(g): “…even if the text of the current provision were in a Rule it would be severely limited in scope: It applies (i) only to conduct by a lawyer that occurs in the course of representing a client, and (ii) only if such conduct is also determined to be ‘prejudicial to the administration of justice.’” (ABA Report to the House of Delegates, Page 2 of Report.)
“Changing the Comment to a black letter rule makes an important statement to our profession and the public that the profession does not tolerate prejudice, bias, discrimination and harassment. It also clearly puts lawyers on notice that refraining from such conduct is more than an illustration in a comment to a rule about the administration of justice. It is a specific requirement.” (ABA Report to the House of Delegates, Page 4 of Report.)