Why would master student spend more than the normal duration to complete master degree?
Here is one more possible reason: the students may be working so much "on the side" that they need more semesters to complete all of the necessary courses.

Not at all universities are students supervised thoroughly or progress is checked regularly, so that working part time and studying part time is possible even when not explicitly studying for a part-time degree.

It is not difficult to imagine that when working 50% of your time and studying 50% of the time, it takes twice as long to complete the course of study. 

Obviously, it depends on the rules and regulations of the university and/or course of study whether this is a possible explanation. As an example, my undergrad course allowed this, and the average duration of study was 85% above the normal duration of study, mainly because so many students studied while working. It was a STEM course. I know of on case where the student actually spent 2/3 of the time working and 1/3 of the time studying.