Because of a bug 5.21.0 was pulled back. You should just use the 5.21.1 version. See also the release notes.
Due to a minor security issue in Mendix 5.21.0, Mendix recommends users convert to version 5.21.1 at their earliest convenience. To facilitate the transition and prevent any potential breaches, the earlier version was retracted from the app store.
For details regarding the bug in question please consult the release notes of 5.21.1: https://world.mendix.com/display/ReleaseNotes/5.21.1