Before we analyze why Henry Ford - a notorious skinflint - would do this, you have to ask yourself why so many other professions and jobs are not unionized. For example, as a Lawyer, I am not part of any union that set wages and working conditions. And yes, wages are low in our profession today - pathetically so. And many "sweatshop" law firms ask young associates to work 60-80 hours a week or more - 10-12 hours a day, seven days a week, or you're fired. Seems like an industry ripe for unionization, eh?
The second reason why many jobs are not unionized is that they are not easily blackmailed by unions into giving concessions such as allowing unionization, standardizing wages, and implementing work rules. A large factory is often mortgaged to the hilt by bankers. The company has to keep cranking out cars, steel, chemicals, or whatnot, just to make payments on the debts and to stay competitive with other companies.
The problem became so acute that Ford did something that really pissed-off his competitors. He more than doubled pay to $5 a day and cut hours to 40 hours per work-week. He wanted a happy, contented, and trained workforce, without a lot of turnover. People who were overworked and underpaid, quit. With higher pay and shorter hours, Ford had one of the most loyal workforces in the industry.
The union succeeded not because they were an idea whose time had come, but because they used strong-arm tactics, muscle, and organized crime connections to force workers to join, and to force management to negotiate. And often management was being sold a product - labor peace - for a price. The unions then told the workers that the only reason they had the pay and benefits they had was due to their efforts. And a lot of people believed this.