Mainly due to the ACA (Obamacare). A small business can't deduct the premium as a pre-tax deduction unless they offer the same insurance plan to all "similar" workers. By paying the premiums after tax and considering them part of your salary, they may be able to get around some of those rules and only provide insurance to selected employees. Now as it happens, many of the schemes for doing this are against the law (ACA) themselves, and the penalties are $100 per employee per day for being non-compliant. So I think it's more than likely that the employer either doesn't understand the benefit of a pre-tax deduction to HIM (as well as to the workers), or the employer is trying to get around some ACA regulations that they shouldn't be doing.
And, for some businesses like small S-corps, if you are both an employee and a significant shareholder, you aren't allowed a pre-tax deduction for medical insurance (although I don't know why).
But I am not expert enough to know all the reasons an employer might do this.
