At a guess I'd say ubiquity and path of least resistance.
Many places I've worked email has been the defacto ad-hoc task distribution system for years.
Thanks! We wanted to remove all barriers of collaboration. It's friction-less using Planleaf, you need nothing other then an email to get going :)
I really like this. I work for an NGO that does work all over the world and I can see something like this being really useful for areas that have bandwidth concerns!