Why do some papers have 10-20 co-authors?
In my field, where author numbers between one and six are the norm, reasons for having more include the following:


The paper results from a large-scale collaboration between different groups that developed different parts of the overall work.
The paper is supposed to give a broad picture overview of something, like in the case of a survey paper or a research roadmap paper. The authors are experts for the different considered areas.
The paper results from a collaborative brainstorming effort. Such papers are sometimes written by the participants of a research meeting (like the famous Dagstuhl seminars), or a breakout group from such.
The paper results from a student project, and most of the co-authors are actually students.