There are many reasons for poor participation. Some people feel they can not afford to save. I know this sounds short sighted. But when you are living from paycheck to paycheck and using your credit card to bridge the long weekends your financial horizon is pretty short. Some never get around to it.
Retirement seems far away, and it is hard to see any urgency to do anything now.
Some are intimidated by all the decisions they personally have to make. They have to decide how much to have withheld from their pay. Then they have to decide what investments to make. It can feel very confusing. The problem is that all these people will eventually grow old and retire, whether they have saved or not.