I’m just guessing it was a combination of (1) an app could be distributed / authorized to only specific email addresses, so far less chance of interruption than you’d get if the website got leaked to the public, and (2) wanting to simplify the reporting logistics by just having people use their private phones, rather than having to set up laptops and internet, and possibly (3) an attempt to stick a toe into the realm of widespread voting by app.

Of course there’s always the possibility it’s a simple matter of (4) someone lobbying politicians into giving money to his brother to slap together an app, because government money.