Apps are expensive to build, allow rampant surveillance that it's very hard to audit under the hood, and mean that customer information is distributed based on your ability to access a smartphone.
The U.K. government also has a poor record of delivering large scale IT projects with complex chains of dependencies (the overhaul of the NHS IT system was cancelled after spending several billions on a Patient Information system that no one ever ended up using). Given the chance, departments would have tried to create the 'Everything App' at enormous expense and with zero interoperability between depts.
The decision to push the GDS methodology across the whole of government was hugely unpopular within the civil service at the time but they've been absolutely vindicated in their approach.