For a very specific reason: app stores log the history of all the app downloads.
This means that someone could check your app download history, and find out that you are using Cryptee. This could put you in a position to give access to your account to a third party.
Cryptee is built to provide privacy and security, with optional deniability in case you find yourself in a power asymmetry. With this in mind, we designed Cryptee as a web app that can be installed without going through any stores. So you can simply un-pin the application from your device, clear your browsing cache and history, and deny having an account if you need to.