Unions in the private sector became much weaker over the last 40 years, but that was not due to the labor laws that empower unions being eased. It was due to the type of industries that unions are prone to form in (e.g. labor-intensive, high volume manufacturing) contracting in the US.
The laws that empower unions make the US inhospitable to key industries.
What's going to happen to the US electric car manufacturing industry if Tesla's workers unionize?
What's going to happen to Amazon's business units if they see massive unionization?
The consequences for a US-based operation when their work force unionizes is a major disincentive for investing in production in the US, and a major impediment to existing US-centred companies from expanding.