You can see the full list in the linked article. In order of importance:
Feeling unhealthy on the veg*n diet
Low identification with veg*nism
Believing society perceives veg*nism negatively
Low autonomy support
Cultural influence making it more difficult to go veg*n
Cost
Weak habit formation around choosing veg*n food
Difficulty finding or preparing veg*n food
Feeling ashamed of one’s veg*n diet
Low personal control over food
Small veg*n network
Feeling that veg*nism hasn’t positively impacted one’s health goals
Low motivation
Frequent cravings for animal products
Specifically, people who felt unhealthy on their veg*n diet were more than three times as likely to abandon it within the first six months (30% vs. 8%). People who did not see veg*nism as part of their personal identity were about twice as likely as others to abandon it (16% vs. 8%). And people who thought society perceives veg*nism negatively were about 1.5 times as likely as others to abandon their diet (13% vs. 8%)
You can see the full list in the linked article. In order of importance: