I guess they don't have enough cash in the bank to keep investing half a million dollars twice a year with no mostly returns for 7+ years (typical time for a start-up to become valuable).
Let's see, if I remember well they invested about 80 000$ in reddit (there was a second investment from YC if I'm not mistaken). So from this, they maybe took 10-15%.
Reddit sold for about 12 million dollars, so that would mean about 1.2-1.8 million: 2-3 years of yc investment.
Of course those numbers are pure speculation and it wouldn't suprise me if I'm completely mistaken...