Culture, policy and structure.
Culture: The US considers public transport to be "for the poor". How many investors and founders in SV take the bus to work? Buses are seen as an outdated mode of transportation. Why create a company for a technology that you expect to be replaced by hyperloop/flying cars/
Policy: Chinas political leadership made a decision that they want electric buses in their cities and that they want them to be built in China. Then, they created the environment that would enable this using directives or subsidies or whatever.
Structure: China is the manufacturing hub of the world. They have good engineers and they know how to make stuff. They have a deep, cheap, local supply chain. Things that the US may not have as much.