Why don't you try the Top up? There is a top up very weekend for legendary weapons, around 115k coins every time.
Because without paypal, or other internet based services: 6000c = $20
115,000kc = $400.
Not alot of people are willing to spend $400 on a virtual weapon. Maybe if the prizes were stackable in those events, some would- but to just get the weapon isn't enough.
You also need to consider that alot of people who play this game don't have jobs (b/c mainly kids/teens play) - asking a parent for around $400, to have one item on a single browser game is insane.