This is a bit like saying "If conventional medicine works then why are people who use it still sick?". Some of the people who use conventional medicine get better, some improve a little, and some don't get better, or get worse. It's the same for acupuncture, though in contrast it's very benign and there's a low rate of adverse outcomes.
Acupuncture can help a lot of people with a lot of different conditions, but especially people with chronic pain. Not all conditions and not all patients are going to respond to treatment equally well, nor are all acupuncturists equally capable.
Medicare now covers treatments for lower back pain with acupuncture because it's been shown to treat lower back pain very well. Up to 20 treatments per year are approved, and if there's signs of improvement, more treatments can be added. So some people are going to get better after less than 20 treatments, some will get better only after more than 20 treatments, some will get only a little better with any number of treatments, and some won't get better at all.