They were not given tax incentives to offer health insurance to employees.
During WWII the government installed wage controls (good old FDR) and companies could not attract new workers. So they started offering health insurance which wasn’t frozen by government. It was government causing the problem in the first place.
It's also a tax-free benefit for an employee. This is also a huge incentive for both the employer and the employee. If your employer gave you $10,000 per year in bonuses, it would be taxable income. The same would be true if the employer paid $10,000 of your rent or mortgage payments every year ... or if they made $10,000 worth of car payments for you every year ... or if they paid $10,000 in life insurance premiums for your every year.
But for some reason health insurance has a special treatment carved out for it under Federal law, which is why companies even offer it these days -- decades after the wage controls of the 1940s were eliminated.