Originally, because Buddhism held that if people cultivated enough virtue over the cycle of rebirth, they would finally reach enlightenment and become Buddhas, there was no significance to any particular death of an individual and hence no ancestor worship, so in the beginning Urabon was an official Buddhist ceremony to revere Shakyamuni.
仏教は本来、輪廻転生し徳を積めば最後は開眼し仏となる教えであり、「特定される個人としての死」はないので先祖崇拝はなく、「盂蘭盆」が正式な仏教行事で釈迦を奉るものである。