It's nonsense to call "The Cage" a colossal failure, since the network liked it enough to commission a second pilot. In fact, the network loved a lot about "The Cage" -- its intelligence, its ideas, its production values. And it was a great success in proving that Desilu, which had been considered a washed-up studio that only produced one sitcom and survived mainly by leasing out its production facilities to others, was capable of producing the most technically elaborate television series ever produced and doing so on a limited budget. But the problem there was that "The Cage" was an all-out, feature-grade production to show what Desilu could do at its best, and therefore it didn't represent the budgetary and logistical needs of a typical episode, which is the sort of thing a pilot is supposed to give a sense of so a seasonal budget can be worked out. "The Cage" (or rather, "The Menagerie," which was its actual title during production) got the network interested enough to move forward, but they needed to see a second pilot representing a more typical production before they could commit to a series.
There were some things about the pilot that they weren't happy with and wanted Roddenberry to improve -- mainly the casting. They weren't happy with the cast and characters. Roddenberry had promised them a progressive, ethnically diverse cast, which was what networks wanted at the time, but had delivered a mostly white cast of largely interchangeable characters. And they weren't happy with GR casting his mistress as the first officer. They were uncomfortable with Spock too, though they eventually got over that. And they did feel it was too sexy and too focused on "cerebral" matters over action.
But that doesn't make it a failure, since they still wanted Roddenberry to show them more, and once he satisfied their concerns, they bought the series. And the first pilot played a key role in making that sale. So it was by no means a "colossal" failure. If it had been, they wouldn't have asked for a second pilot, Star Trek would never have made it on the air, and we would've had to settle for being fans of, I dunno, The Invaders or The Six Million Dollar Man. At worst, it was a minimal failure, close enough to success that it kept the door open and paved the way for subsequent success.
