What I've seen in the Netherlands, where unions were traditionally quite strong, is that for the last 2 decades or so a sentiment has been spreading that they are old-fashioned, from a different era, and somehow more relevant to people in blue collar jobs.
Esp. in IT there's a false sense of job security, and a kind of individualism and privilege by 'spoiled' employees used to great demand for their work, the nicest perks, and job-hopping very normal. So why join a union, right?