It was actually the highest-grossing film of the year, but was still considered an unsuccessful film because it still showed a loss even after a successful run.
The idea that Cleopatra was "unsuccessful" is a myth that simply should not be perpetuated.
Not only was it the highest grossing film of the year, it set many box office records throughout the world. The film turned a profit in the early 1970's in an era when films were expected to take a while to play out.
It has continued to make money ever since. How many films that are 50 years old have a theatrical reissue at a fairly large number of locations?
Cleopatra clearly destabilized Fox for a few years, but only a part of this can be blamed on the production itself. Fox shut down productions that probably should have continued. Due to the shutdown, general studio costs were charged off to Cleopatra, causing the putative budget to be greatly increased. The joke was that if a studio exec took his mistress to dinner in Beverly Hills, the cost was expensed to Cleopatra.
A lot of this was general mismanagement and had little to do with what was happening in Rome.
Darryl F clearly demonstrated this by 1) delivering The Longest Day at a critical time, and 2) finishing and releasing Cleopatra, and 3) getting other projects into production.
Like the marvelous fading frescoes that provide the interfaces to Cleopatra's scenes, we are now left with only the film itself. "Unsuccessful"? I think not. Flawed perhaps but still a remarkable effort from many technical and esthetic points of view.