By distributing new coins via mining, miners are motivated because they can compete to gain a more competitive ranking in the coin ownership. If the PoW is cpu-only as Bitcoin was when half of its coins were mined, can potentially get the entire population excited about mining.
How can PoS attain similar marketing and adoption?
ASIC resistant coins like Protoshares for example just end up with people running server farms of hyperthreaded multicore CPUs. In the end you end up with the same problem though perhaps not as stark.