That's the long and the short of it.

For reasons of his own (probably protection against a subordinate turning rat), Vito had only two caporegimes. No one else got close to him except Tom, and all orders came through Tom. And, as the novel tells us, he kept Tessio on a long leash, far from the Mall (where Clemenza lived), partly to keep the two from conspiring against him. Naming Tessio consigliere probably would have gone down badly with Clemenza--he needed both of them, just where they were in the family.

I think Vito knew that Tom, as a non-Sicilian, lacked certain important attributes that Genco, as a Sicilian, brought to the position. But I think his long-term goal was for the family to be "legitimate." Tom was a lawyer--"legitimate." More important, since Sonny was the heir-apparent, Vito believed that Tom could rein in Sonny's hot-headedness.

Obviously, succession-planning was not Vito's long suit.