Because investing time and money in a channel (email) that most people consider as that tiny room in their house nobody ever opens, full of unorganised crap you should have trashed years ago but never did, is a huge ask. We can probably all live with that type of mess in our lives. It is stressful, it is impractical, it is clunky. But we can still go there once or twice a day to see if anything worthy happened (usually not), and then go back to using more modern and comfortable means of communication for the majority of our interactions. Who cares if there are 1,346 unread messages, junk messages, when we will never ever pay any attention to them?
And so, Hey.com might just be misplaced in time, based on the assumption that we actually care while we do not. There is no great product, no flawless on-boarding, no inspiring mission that can revert this.