The reason being that you need to be connected to internet to use it as it renders the music over the net and this is for me a deal breaker as first of all only have a limited amount of internet in the contract for my devices and second being connected all the time drains the batteries really fast...and just to have to depend on having an internet connection to make the music is slightly annoying but now they are going to release a new application and it seems that the Beta is ready for release soon.
Looking at the pictures it seems like it would have a little bit hard time competing with some other complete applications that exists but it also seems that it has some neat tricks on it's own. It is also clear on their forum that it is not going to replace PocketBand Pro but being a separate app with it's own identity.