I agree. I don't want to take power from the existing unions, but they should be able to exist in some less official capacity as well. 1st amendment says freedom of association, right?

Because employers have the power. Without the legally protected status as a union you have no legal right to protest the conditions of your job. You have no right to refuse to work under dangerous conditions. And employers are free to retaliate against workers for even talking about unions or talking about reporting the dangerous conditions. How are you going to get people together who are all desperate for money and get them all to agree to go on strike and then get other people to not come in and take the jobs. One way used to be to call them scabs, make everyone else hate them through propaganda campaigns, and hire the Mafia to beat them up until they quit and no one else would take the jobs. It wasn't until the NLRA that unions were protected.

But conservatives have turned anti-worker now due to their reliance on corporate donations among other things. And they have spent decades making unions look bad, saying they're just criminal organizations, and calling them communism. So not enough people are going to feel sorry for the striking worker or hate on the scabs enough to pressure the companies to give in to demands.

Taxi workers are no longer considered independent contractors.

Independent contractors aren't covered by the NLRA. So they don't have the right to have a union and a company is not required to deal with a union and can retaliate against independent contractors for even discussing being in a union.

Now what is considered to be an independent contractor has varied over time. Generally when conservatives are in office, the definition expands to exclude more people from protection. And more liberal presidents put in more liberal heads of the NLRB that are more likely to shrink the definition.

There were several factors that led to the taxi workers alliance creation and becoming recognized as a union. One of the biggest ones is that they used the medallion system and other similar laws to their advantage in that if enough of the medallion owners were refusing to work, then they couldn't just hire new workers. There aren't exclusivity laws like that in very many other industries, including delivery services, to use to put pressure on employers.

And it took like 20 years to be recognized as a union when the NLRB declared that taxi workers are employees and not independent contractors in 2015.