Not wanting to support the cruel treatment of animals is likely the top reason. I assume these are also often the same people who eat a vegan diet because it has a significantly smaller negative impact on the earth.
And then there's health reasons. Animal products tend to be highly processed. Plus some people just don't digest them well and it has a negative impact on their overall health. There are also people who just think a plant-based diet is all around healthier for various other reasons.
So, guilt/wanting to live more ethically, and improved health are probably what motivates people to adopt a vegan lifestyle thd most.
