Demand and supply.
They rarely bring back boxes that has the most valuable gears and the people that can afford buying multiple boxes hoard them to sell at a higher price later. Let's say only 40% of the population buys KP and most of them hoard the gears then that's your answer. And at the rate of people quitting faster than the people trying out the game then it's a lose-lose for both the players and WP.
How can you sell more items if the only buyers that can afford them are the same people who wants these gears at a high price because of hoarding? They only cater for the people that has the zeny and not for the first time players who doesn't know how to get zeny or has the time to do so. So what do those people do? Either buy KP or quit.
We've come full circle.