I wonder how much of that is due to habits and regulations that are necessary in the absence of basic income. When most people are full-time workers, and when most people need to be in order to survive, it's not surprising that the entire economic system gets optimized for full-time work.
Lots of government regulations are also designed with full-time workers in mind, with the assumption that most part-time workers want to "upgrade" to full-time a.s.a.p. For example, in some jurisdictions it is illegal for an employer to employ a part-time worker for more than X hours a week (nearly full-time) without giving them benefits equivalent to what full-time workers get. This makes it very difficult for an employer to maintain an ongoing relationship with someone who prefers to work 20 hours a week. Not enough hours to justify full-time employment but too many hours to keep on the part-time payroll.
Policies like that are necessary to protect workers in a world without basic income. In the long term, I imagine that a guaranteed basic income would make a lot of current employment-related regulations unnecessary, leading to more flexible terms of employment. But it will probably take a decade or more for Switzerland's economic system to adjust to basic income, and I really hope that the pain of the transition period doesn't make them call it a failure too soon. (I can already see the FOX News headline: 3 years after Switzerland implements basic income, their unemployment rate is up 20%! See what happens when Commies take over your country! etc. etc.)