Short answer: 60% of all DASH is stored in masternodes which generate an 8.5 DASH per month dividend for their owners. Most of these masternode owners are long-term holders who really enjoy getting paid a 9% annual ROI for doing nothing but running a node.
More people want to buy DASH, but these masternode owners are not being enticed to sell their holdings, meaning you have more buyers competing for very few sellers. As the price rises, maybe owners of DASH will be enticed to sell their holdings. Considering that many of us think DASH is worth way more than $70 each, it could take awhile...
There is a small problem - all master node owners won't be able to redeem their Dash. Because if they try to do this, the price will dump to the bottom.