It's not that Abe Fortas was an unqualified candidate. As an associate justice of the Supreme Court, he was capable of filling the role as chief justice. Unfortunately, some bad moves by Lyndon B. Johnson, the president who nominated him to the position in 1968, doomed his chances.
Johnson erroneously assumed that Fortas would be confirmed with little controversy. After all, he was counting on the support of his Senate mentor, Richard Russell of Georgia, as well as that of Republican Minority Leader Everett Dirksen. Unfortunately, Johnson had managed to irritate fellow Democrat Russell by failing to deliver a federal judicial appointment for one of Russell's supporters. That, combined with appointing an unqualified friend to fill Fortas' associate seat once his nomination was confirmed, was too much presumptive arrogance for the Senate to bear. The body rejected Fortas' nomination, which would prove to be a good move later on. Fortas maintained his original associate justice seat until he was forced to resign early on in the Nixon administration under allegations of questionable financial dealings.