It's primarily because corporations like Walmart are known for union-busting and create a fair amount of anti-union media to stir up controversy about them.

Unions aren't liked by big companies because it leads to better pay, stricter labor rules, and more benefits for workers, which translates to less profits for companies and their shareholders. Not having a union also allows companies to be a lot harsher on employees because there is little risk of an organized strike.

Union shops are also prone to following strict rules like shift ends, overtime, and who is allowed to do what, which can be incredibly frustrating when dealing with them.

It can make for an in-flexible organization making it very difficult to accomplish projects ahead of schedule or delivering excellence.

Unions can get in the way of hiring talented people from the outside in favor of promoting within, which is good for existing employees, but bad for the organization as a whole.

If you are a star employee you will often find yourself taking a backseat to long time employees. The Union favors seniority (gotta put in your time) and strict rules about pay grades, promotions, and seniority can hold back really talented peoples careers.

Unions are great for protecting the average worker, meaning they are great for mediocrity and large pools of average workers, but aren't so great for smaller companies and super talented employees that can do better at a more flexible company.