Matthew Panzarino, writing at TechCrunch:
The project was started two weeks ago by engineers from both companies. One of the reasons the companies got involved is that there is poor interoperability between systems on various manufacturer’s devices. With contact tracing, every time you fragment a system like this between multiple apps, you limit its effectiveness greatly. You need a massive amount of adoption in one system for contact tracing to work well.
At the same time, you run into technical problems like Bluetooth power suck, privacy concerns about centralized data collection and the sheer effort it takes to get enough people to install the apps to be effective.