Why are there different Buddhist traditions?

Buddhism was founded more than 2,500 years ago, and with this long passage of time, three main traditions and four smaller schools have evolved. These developments took place as Buddhism adapted to the different countries, conditions and cultures it spread to.
However, the Buddha's Teachings have proved to be very resilient as while the outer trappings may be dissimilar, the core Buddhist doctrines remain the same among the various traditions. For example, the acceptance of the core doctrines, or "Unifying Points", between the different traditions was formally endorsed by the World Buddhist Sangha Council in Sri Lanka in 1966.
Buddhists accept and respect diversity, and consider the various traditions merely as different routes to the same destination.

The Theravada tradition is the oldest and most conservative. It is the closest to the original form of Buddhism as taught by the Buddha Himself. It is simpler than the other traditions in approach with few ceremonies and rituals, preferring instead to stress on discipline and morality and the practice of meditation.

The Mahayana tradition started to develop in India between 200 B.C. and 100 A.D. It has adapted to different Asian cultures absorbing elements of Hinduism and Taoism. Mahayana Buddhism emphasizes very much on compassion and faith with the goal of helping all others attain enlightenment. The Zen, Nichiren and Pureland sects are offshoots of Mahayana Buddhism.

The Vajrayana or Tibetan tradition rose around 300 A.D. in Tibet when Buddhist Indian monks brought over a brand of Buddhism with tantric practices. This combined with elements of the local Bon religion gives Vajrayana its unique practices. It tends to be heavier on rituals, mantra chanting and visualizations. The most well-known face of Buddhism, the Dalai Lama, is the spiritual head of the Vajrayana tradition.