I'd guess that unions aren't very effective unless a substantial percentage of the workforce joins, so it makes sense for an organization with limited resources

I'd guess that unions aren't very effective unless a substantial percentage of the workforce joins, so it makes sense for an organization with limited resources to try to focus on getting wide adoption in a small, specific part of the tech industry rather than trying to get everyone to join all at once. Maybe the game industry is the best place to start a tech union as a proof-of-concept, given that (if the stereotypes of game studios are true) it has a higher proportion of employees aren't too happy about their relationship with their employer than other parts of the tech industry where the pay and working conditions are better.