The first reason is probably just that manufacturing stuff is hard, especially at scale like this. It takes a while to get good at it, and develop processes and talent to ensure production lines move along smoothly.

The second reason is that I think they invested heavily in an ultra-automated factory, and they weren't able to get that working quite as well as they'd hoped in the time allotted.

These are sort of the high-level reasons, maybe somebody with more specific knowledge could give a more nitty-gritty explanation.