According to Mahayana Buddhists, the Buddha was a vegetarian. The Mahaparinirvana Sutra teaches that he taught against eating meat:
Then Maha-Kasyapaika-gotra asked, “If it is very important to uphold the impropriety of meat-eating, would it not then be wrong to give meat to those who do not want meat?” [The question here is whether it is wrong to give meat to monks who do not want it, which by the Buddha’s reckoning should be every Buddhist monk, but clearly the implication extends to forcing one’s own food choices onto others, particularly their own children, who are vegans by nature—whence the practice of carnism ultimately originates.] [The Buddha replied:] “Excellent, noble son, excellent! You have understood my intention. One who protects the authentic Dharma should not do that. Noble son, henceforth I do not permit my disciples to eat meat. If I have said that [one should view] the country’s alms-food as the flesh of one’s son, how could I permit the eating of meat? I teach that the eating of meat cuts off maha-maitri.”
“Blessed One, why did you permit the eating of meat that was blameless in three respects?”
“Because I stipulated these three types of blameless as a provisional basis of training; I now discard them.”
“Blessed One, what was your intention in talking of the ninefold great benefit and the abandoning of the ten types of meat?”
“Because those pronouncements were stipulated to restrict the eating of meat; they are also withdrawn.”
“Blessed One, what was your intention in stating that meat and fish are wholesome foodstuffs?”
“I did not say that meat and fish are wholesome foodstuffs, but I have said that sugar-cane, winter-rice, ordinary rice, wheat, barley, green lentils, black lentils, molasses, sugar, honey, ghee, milk and sesame oil are wholesome foodstuffs. If I have taught that even the various garments for covering the body should be dyed an unattractive color, then how much more so attachment to the taste of meat foods!”
“In that case, does it not follow that the five milk products, sesame, sesame oil, sugar-cane sap, conch-shell, silk and so forth also violate the precepts?”
“Don’t cleave to the views of the Nirgranthas! I have imposed the bases of training upon you with a different intention: I stipulate that you should not even eat meat blameless in the three respects. Even those meats other than the ten [previously forbidden] kinds should be abandoned. The meat of corpses should also be abandoned. All creatures sense the odor and are frightened by meat-eaters, no matter if they are moving around or resting. If a person eats asafoetida or garlic, everybody else feels uncomfortable and alienated—whether in a crowd of many people or in the midst of many creatures, they all know that that person has eaten them. Similarly, all creatures can recognize a person who eats meat and, when they catch the odor, they are frightened by the terror of death. Wherever that person roams, the beings in the waters, on dry land or in the sky are frightened. Thinking that they will be killed by that person, they even swoon or die. For these reasons, Bodhisattva-mahasattvas do not eat meat. Even though they may appear to eat meat on account of those to be converted, since they do not actually eat ordinary food, then how much less so meat! Noble son, when many hundreds of years have elapsed after I have gone, there will be no stream-enterers, once-returners, non-returners or arhats. In the age of the Dharma’s decline, there will be monks who preserve the vinaya and abhidharma and who have a multitude of rituals, but who also look after their physical well-being, who highly esteem various kinds of meat, whose humors are disturbed, who are troubled by hunger and thirst, whose clothing looks a fright, who have robes with splashes of colour like a cowherd or a fowler, who behave like cats, who assert that they are arhats, who are pained by many hurts, whose bodies will be soiled with their own feces and urine, who dress themselves well as though they were munis, who dress themselves as shramana, though they are not, and who hold spurious writings to be the authentic Dharma. These people destroy what I have devised—the vinaya, rites, comportment and the authentic utterances that free and liberate one from attachment to what is improper, selecting and reciting passages from each of the sutras according to their inclinations. Thus there will appear [so-called] shramana, sons of Shakyamuni [so-called Buddhists], who will claim that, ‘According to our vinaya, the Blessed One has said that alms of meat-stuffs are acceptable’ and who will concoct their own [scriptures] and contradict each other.
“Moreover, noble son, there will also be those who accept raw cereals, meat and fish, do their own cooking and [stockpile] pots of sesame oil; who frequent leather-makers, parasol-makers and royalty ... The person I call a monk is one who abandons those things.”
“Blessed One, what should be done by monks, nuns, upasakas and upasikas, who depend upon what is offered to them, to purify alms-food that contains meat in such places where the food has not been verified?”
“Noble son, I have taught that it does not contradict the vinaya in any way if they wash it [the vegetarian food, after the meat has been picked out or it has been verified that it had none] with water and then eat it. If it appears that the food in such places contains a lot of prepared meat, it should be rejected. There is no fault if one vessel touches another but the food is not actually mixed together. I say that even meat, fish, game, dried hooves and scraps of meat left over by others constitute an infraction. Previously, I taught this in cases arising from the needs of the situation. Now, on this occasion, I teach the harm arising from meat-eating. Being the time when I shall pass into Parinirvana, this is a comprehensive declaration.” Mahaparinirvana Sutra
So I don't see why vegetarianism is not a common practice among Buddhists as it is in Jainism.