So many Buddhist schools! Why does Buddhism have so many variations, like Zen, Nembutsu, True world, Nichiren Buddhism, etc.? Why is Buddhist philosophy not uniform?
Buddhism changes as it goes into different cultures and adapts and is influenced by local and preexisting culture. It changed when it went from Indian into China and again into Japan and again into Tibet. I believe it is changing still as it goes into the West. For instance my group the Triratna Community is an explicit attempt to bring Buddhism to the West in an ecumenical way.

To follow on from that, the Trirantna's founder Sangharakshita identified the act of going for refuge (to the Buddha, Dharma and the Sangha) as the central act of all Buddhist schools and the principle that can (and does for us) underpin any Buddhist movement. So our interpretation is that all schools have the same principle so are not as divergent as one might think at first glance. Of course I'm not banging the drum for that specifically here and people are obviously more than welcome to agree or disagree on that interpretation.

Buddhism also does change within a culture by reacting to the Buddhist school before it. The most notable example I guess is the growth of Mahayana as a reaction against the perceived ossification of the Hinayana (though this term is derogatory) schools. Parallels can easily be seen in other religions such as the Protestant Reformation, the Cathars, Lollards etc...

Also I might quibble with the premise of the question itself. I'm not convinced that Buddhism does have a particular high amount of branches as compared to other religion. It's just that we are not used to the religion. Also if we don't identify culturally with any particular branch we see them all. I think that Christianity has a lot of sub divisions - Protestant, Catholic, Greek Orthodox, Methodist, Church of England etc.. Also having listened to Peter Adamson's excellent podcast of the history of philosophy, I find the branches of Islam bewildering.

This is probably a bit TR;DR. Sorry