The idea will have been pitched to Gillette by an advertising agency, who would be aggressively trying to sell the idea in. I can almost hear the conversations which will have occurred: "It's a brave idea. Some people won't like it. But someone needs to take a stand - and Gillette is the brand to do that." And then when the backlash from the initial ad started to hit: "Of course there's backlash. But we need to stand our ground. That truly is the meaning of bravery, standing firm in the face of adversity. Speaking of which, here's a fat acceptance follow-up ad, and a transsexual ad too."

The agency would have likely known after the reaction to the first ad that the whole campaign was doomed, but the opportunity to sell in two more ads to P&G would have been too tempting to ignore. Agencies are struggling to make money nowadays, with the decline of TV. There isn't nearly as much profit to be made from making online banners as TV ads. So they aggressively try to sell-in "video content" which is basically the same thing as TV ads, but they also get to make extended 60-second / 90-second versions, which is where they make bank.

A valid question could be that wouldn't the agency be better off being honest with their client in the hopes of ensuing a long-lasting relationship, rather than taking the quick cash on a doomed campaign? The reality is the industry nowadays is ruled by short-termism, and these big businesses like P&G swap out their agencies on a regular basis anyway. So there's no incentive for agencies to actually build a trusting relationship with their client, it's all focused on the quick sale. They're going to get fired anyway.

Of course in the case of Gillette, none of this excuses the client themselves for letting a campaign like this go to market, let alone continue it when it is so obviously being poorly received. If the CMO can't spot when the agency is trying to sell them a shit idea, and shut it down immediately, then they have no business being CMO. I can only assume they got so caught up in the echo chamber created by the Gillette marketing team and their agency partners, that they decided the risk was worth it. Certainly they can't have anticipated the outcome would be this bad.

Final thing - fame is a very real thing in the marketing/advertising industry. Certain ad campaigns blow up, people love them, they win awards at Cannes, and for a while the marketing team behind the campaign become like celebrities within the industry. Being behind a campaign like that can make careers. I suspect the Gillette team believed this campaign would make them famous. Unfortunately for them it did, but not in the way they would have hoped.