Well, there's noone in the novel (or film) other than Tom that Puzo or FFC presented as having much of an opportunity to be considered by Vito to fill that vital post. While Tessio did have some consigliere characteristics, Tom was family who came with the indigenous loyalty that comes from being raised in a father's household. While legal education and training is not a consigliere requirement, it wouldn't hurt. Also, it fit in with Vito's plans for the future of the family.