I've had some experience starting new manufacturing facilities and scaling up processes. It takes time and there are lots of pitfalls. So even though they claim a facility for producing 150,000/yr, it may be with three shifts, and multiple lines that aren't fully staffed or equipped. So if they had five lines, 3 shifts at around 150k, then with 1 shift and a single line, you've only got 10k/yr. Throw in another products being built on the same line(s), and you've got lots of reasons for lower production.
That said, I've also not found a single sales/marking person (from CEO, VP, down to the guy with his first job) who can accurately predict how much "product" is needed in a given week, month, year. Add to this a degrading product (not quickly mind you, but still), and you need to tread carefully on how much extra product you keep in the pipeline. Ultimately, it's manufacturing's job to put up the numbers that the upper management believe they can sell. From this side of the fence, it sure feels like they are in a management imposed slow down, for whatever reason. We can only speculate: 1) costly manufacturing (we saw this excuse when built in Japan), future product/battery improvements (if only), limited resources (don't want to harm their ICE business), limited parts (we've heard this from Tesla about batteries and maybe Nissan is seeing the same thing), poor short term planning (didn't give one part vendor enough time to scale-up), inflexibility of production-delivery chain (we saw this with the initial Leaf out of Japan, it seemed to take more than a month to deliver to customers).