One reason for Buddhism’s diversity is that Buddhist doctrine allows for different approaches to teaching to match different conditions in each locale
Thus Buddhist sects teach multiple ways to reach nirvana (liberation from the cycle of rebirth). This sense of multiple ways to attain nirvana is similar to Hinduism’s acknowledgement of multiple paths to moksha (release from rebirth).
Also, according to Buddhist lore, the Buddha himself taught that no single canon (established rules) should govern all Buddhism. Instead, Buddhist teaching should adapt to varying conditions.