PIVX Masternodes require 10,000 coins. For every Masternode in use it means 10,000 coins are taken out of circulation to service running it. There is a limited supply of Masternodes... Right now there is about 53,000,000 coins and so right now the maximum number of Masternodes possible is limited to about 5,300. (53,000,000/10,000)
Masternode growth is limited to coin supply growth which will be 5 coins per minute (block time). But since coins are distributed all around in smaller amounts, there can't really be 5,300 Masternodes because not enough coins for everybody to set up a Masternode. You must work to achieve the masternode. This gives it value and backs its own value. This whole design perpetuates the rarity exponentially because as more Masternodes are created the coin gets even more valuable which drives up the benefits of having a Masternode. This in turn then continues to drive up the demand even more for people to achieve the 10,000 and take them out of circulation. But every time this happens the coin will only get rarer and more valuable and it is a never ending cycle of perpetual wealth generation for everyone holding the PIVX coin.
Why? Because if you can achieve 10,000 coins, and create the masternode, you are then guaranteed to find a block at least once per week. PIVX is basically designed to pay you to buy it and save it and take its coins out of circulation. It is the true saver's coin. It's a win win because even if you don't have a Masternode yet, we all benefit just owning any coins at all we will catch the ride and increase in value.