It is not about money, tax is about money. This is about the extra work needed to accommodate for said obligations, it can involve several full time jobs, and the state considers that it is too much for a company of less than 50 employees.
Really, it is a lot of paperwork, and the more a company spends on paperwork, the less there is to pay employees.
Public perception. Small business owners occupy a different perceptual space than large businesses. Lawmakers don't want to be perceived as hurting small businesses and craft laws to avoid it. It's more about maintaining political power as much as anything else.
I think a system where we go by revenue is a lot of logical than going by number of employees. If a business makes 1Bpy but only has two employees, would it not be exempt from the current regulations?