If I’ve learned one thing from the cleantech funding bust, it is that many of these would be more successfully funded as basic research than as startups. The iteration and development timelines and sales cycles don’t really fit the standard venture fundable startup model.
Part of a CFO's job is finding the cheapest capital source for the companies growth, sometimes that raising private equity and sometimes that is raising public equity (from IPOs).
Also, being public is a resource drain on the organization. You have to comply with SEC guidelines, employees have to be watched for insider trading, all of your company financials and other details are now public for the world to see. Sometimes the burden doesn't make sense for a high growth company.