Why doesn't it drop?

I wanted to change my scythe so I opened MappingOut, found the one I wanted and went to kill the boss only to find the weapon didn't drop. Is this chance based? If so, what kind of chance is there for a boss to actually drop a green weapon?

It's a random chance to drop. Won't always drop when the boss is killed. Which means if it doesn't drop, your only choice is to map out and map back in and kill the boss again and again till it drops.

As for the chance.. try going through this page.. it might or might not help you:

http://guildwars.wikia.com/wiki/Drop_rate

The best way is to leave the boss for last kill everything around it then flag heros/hench off of radar.

I don't get it... what does this accomplish?

It means that you have better chance at getting it instead of your npcs.They get drops to and it could be going to them.When placed off radar you stad a better chance at getting it.

If they're far enough, they won't get any drop, so you get the full chance of receiving that green (unique items are exempt from loot scaling, so the chance of a green to drop doesn't increase with the number of party members).

Thank you, that clears things up.

The chance of the green item dropping is very small so you may need to repeat many times also some seem to drop more frequently that others.
I doubt anyone has the full accurate info on how this works in the game.

Some bosses I have repeated solo dozens of times and had a very good drop rate while others have never dropped once.
I appreciate its a random chance and in theory you could kill a boss a thousands of times and get nothing but cannot help thinking there is a better system than random chance.

Would be nice if you began with a reasonably high chance that dropped astronomically each successful run.
Chances of having the item for your own use would be good chances of finding s second one practically none.

That would be more realistic you kill him and take the trophy he gets resurrected but unless he gets another item made you never get another drop for you.