If anyone believes that, it's "I've got a bridge to sell you" time. The real reason was to use it as an excuse to blame the gun trade, to generate anti-gun publicity, and to tip the public into accepting stricter gun laws nationally.
Another possibility is that it was an excuse to arm some of the cartels, notably Sinaloa, that Obama had some relationships with. Frankly, I discount that one, because I think that the cartels have other sources of arms and don't need them acquire them in that fashion. But, if US dealers could be blamed for cartel misdeeds in Mexico, it would, it was hoped, guilt the public into accepting stricter laws.
You got it, the whole scam was an attempt to bolster the “90% of cartel guns come from US gun stores” like that PIAPS and Øbowel had been screeching.