The idea of "Veganuary" reinforces the misconception that veganism is a diet that people can just try and ditch at their convenience.
This weakens a whole movement intended to elevate the moral status of non-human sentient individuals so that they're finally free from the human notion that they are on this planet at humans' disposal.
The fact that their only movement, based on the principle against all animal use and exploitation for human ends, is trivialized and reduced to a fleeting diet by the very same people who claim to hold this stance is a disservice to the animals.