[alexhawdon]

I believe that's because their original focus, and still the primary focus of the mobile app, is 'hyperlocal', near-term precipitation prediction.

To do this they require access to high-resolution (spatial and temporal) rain radar data, which isn't universally available. (In the USA this is provided by the NOAA, in the UK by the Met Office.)

[clashmoore]

Hyperlocal because it also uses the data from many small private weather stations around the US. So many times the data you're seeing is from a weather station in a neighbors yard around the block.