This is a pretty commonly asked question. The answer is two fold:
1. putting the comma before doesn't actually make commenting out columns easier - it just moves the problem to the other end of the list. Just try commenting out `first_name` in your example and you'll see what I mean.
2. because normal convention would be for the comma to come immediately after an item in written English.
Re; #1, I also use comma-first indenting and I actually put each field in it's own line specifically for ease in commenting (first_name would have its own line)
1. putting the comma before doesn't actually make commenting out columns easier - it just moves the problem to the other end of the list. Just try commenting out `first_name` in your example and you'll see what I mean.
2. because normal convention would be for the comma to come immediately after an item in written English.