Passage: Many people with asthma, like those with other chronic disorders, use alternative treatments; surveys show that roughly 50% use some form of unconventional therapy. There is little data to support the effectiveness of most of these therapies.  Evidence is insufficient to support the usage of vitamin C. There is tentative support for its use in exercise induced bronchospasm. In people with mild to moderate asthma, treatment with vitamin D supplementation is likely to reduce the risk of asthma exacerbations.Acupuncture is not recommended for the treatment as there is insufficient evidence to support its use. Air ionisers show no evidence that they improve asthma symptoms or benefit lung function; this applied equally to positive and negative ion generators.Manual therapies, including osteopathic, chiropractic, physiotherapeutic and respiratory therapeutic maneuvers, have insufficient evidence to support their use in treating asthma.  The Buteyko breathing technique for controlling hyperventilation may result in a reduction in medication use; however, the technique does not have any effect on lung function.  Thus an expert panel felt that evidence was insufficient to support its use.

Question: Why expert could use the evidence?
insufficient to support its use