Mailbox had multiple causes of death. Most people don’t download email apps; those who do don’t want to pay for them. The only path forward for an email app is acquisition, and the fate of most acquired apps is death. Other email apps copied Mailbox’s best features; aside from a somewhat dubious automatic archiving feature, Mailbox stopped building... See more

The consensus view, then as now, is that the content was not good, it was not shareable (no screenshots!), and that Quibi blew through too much money too fast, before they could figure out what they were doing. But also, they spent too little money to effectively compete with Netflix, HBO, Apple, Disney, etc.