Not surprisingly, the Census data suggest well-paid workers find it easier to save than the lowest-paid. But income isn’t the only factor. Eligibility is also a major issue for part-time workers and people who change jobs frequently. Companies often require employees to work for a certain amount of time before they can sign up for a 401(k), and employers aren’t required to allow part-time workers into a plan until they’ve worked 1,000 hours during the previous year. 

https://www.bloomberg.com/news/articles/2017-02-21/two-thirds-of-americans-aren-t-putting-money-in-their-401-k