It was the easiest choice. In fact the only choice. With Genco dead, and Clemenza and Tessio running operations, the Don had no other choice. Sonny was the heir aparent, Michael didn't want in tht family business, and Luca probably wasn't consigliere material. That left Tom.
In the novel it is mentioned that Vito insisted that Tom studies law. So it had been a plan from Vito that Tom will be a consigliere, either to him or to Sonny.
