Because it's a push rather than a pull medium (you don't have to remember to check it) but asynchronous (you don't have to check it immediately). And one that

Because it's a push rather than a pull medium (you don't have to remember to check it) but asynchronous (you don't have to check it immediately). And one that everyone already has access to (you don't have to create Yet Another Account on Yet Another Service to keep up with questions, answers, and new development).