It's a fair question; the answer to that doesn't seem to have been communicated yet. We can speculate as to reasons why: one could be that the authors feel tha

It's a fair question; the answer to that doesn't seem to have been communicated yet. We can speculate as to reasons why: one could be that the authors feel that the decentralised approach is lacking some important feature that could save lives, such as insufficient feedback about how it is being used. Another could be that they feel that they can implement a solution faster than waiting for Google/Apple to finish their platform and then building something on top of it. This leaves open the possibility of changing tack if the new API is superior. Or maybe they're just hedging their bets in case the Google/Apple approach doesn't work or gets pulled. (I'm assuming none of this is a nefarious plot by the government to add to their general surveillance powers. I personally don't think they'd care given the considerable abilities they already reputedly have to track mobiles).