A disciples also asked Buddha, ' How come the three kinds of clean flesh were allowed to eat during the first turning of the wheel of Dharma, but not now?" The Buddha replied, " Precepts are like stairs going up one step at a time. During that time, some people who had the chance and the capacity to learn Buddhism came for the teaching. If asking them not to eat any meat right away, which they were unable to comply, would have created obstacles to their practice.". So, out of compassion, the Buddha initially permitted them to eat three kinds of clean fresh. Afterward, through gradual guidance, they were led to quit meat altogether. 

~Depicted from The Right View