But everyone can not follow that vow. We have to take food in order to survive and sustain this body. Hence, we need to take that path which is less sinful and does less harm to other jivas (living entities).
In other plants, like mango, coconut, etc., by plucking the fruit, we are not killing the plants, and so we are doing very minimal or no sin at all. So vegetarian food is less sinful. Moreover, it is inevitable for our survival, but non-vegetarian food is a luxury to us and we can survive even if we avoid that.It is more sinful since we need to kill animals all the time.
Whenever such “sadhana” opportunities are cut short, it becomes a sin. For plants there is not much “sadhana” available to them. They cannot do any kind of physical or mental activities in terms of “sadhana”. So by cutting them, we are not doing much harm to their “sadhana”.So it is not so sinful.
But animals can do a good amount of “sadhana” by means of physical and mental activities. By killing them, we are cutting short their opportunities to do “sadhana” towards their liberation (moksha). Thus it is much more sinful.