Because Microsoft's Exchange is to email as IE 6 was to HTTP/s. It is by far the most popular[0] mail server in the world and more to the point it is used by a lot of influential organisations (government, accademia, et al).<p>However Exchange hasn't done anything innovative in at least ten years and if Exchange doesn't move forward then nothing else does either, as everything else is aiming to be compatible with Exchange and little else.<p>If Google (Gmail) and Microsoft (Outlook.com, Exchange, Office 365, et al) came out tomorrow with a way to progress email into the future it WOULD happen. It wouldn't happen overnight (unless backwards compatibility was assured) but it would eventually happen.<p>If you could get Google, Microsoft, and the OSS crowd onboard (Postfix, Sendmail, et al) then it would be even more successful and we might be onto something new within five years give or take (as most of the webmail would migrate, Exchange, and so on).<p>But Microsoft and Google are businesses, and why would they invest capital into new-email when it won't generate revenue? Exchange/Office 356 is going to sell with or without this as is GApps.<p>[0] <a href="http://www.stillhq.com/research/smtp/survey/poster-lisa2007.html" rel="nofollow">http://www.stillhq.com/research/smtp/survey/poster-lisa2007....</a>