If you have an extra comma in the end of the last line it will work in some browsers but not in all browsers. Making the error harder to detect than a extra comma at the beginning (which fails on all browsers). And most developers prefer to see the error right away (so they can fix it), instead of risking a production issue for inadvertently not supporting some browsers. Especially if the solution is as easy as removing a comma.
Plus, having the comma at the beginning of the line, make it simpler to add a line at the end and you will have to touch only that line (you will not need to add the comma in the line before). Which is important if you are using version control (e.g. diff, annotate, bisect). Someone can argue that adding a line at beginning of the array or object will need the same extra work of touching 2 lines (if you use commas at the beginning), but in my experience, inserting a line at the beginning is much less likely that inserting a line at the end.
