With every app you download - Another username/password and another potential security vulnerability - More complexity to your life as you have to familiarize

With every app you download - Another username/password and another potential security vulnerability - More complexity to your life as you have to familiarize yourself with another app - More space on your phone Maybe your app would be an improvement over current solutions but it's not going to be a significant enough of a job to help you get that initial critical mass of users.