The root of it all is --
-- there are many excellent mail apps these days, and most of them come from "proper" companies, with funding which allows for more than one developer, support people, testers. For cloud based apps (which are getting more and more popular), money to cover server costs, server developers.
Someone (usually QA) to work out "what mail service break this week, and how exactly is it broken now". And let's not forget marketing and business development (Sparrow, Mailbox, Accompli...)
The others -- K9 with its 100+ contributors (I know they're not all equal, but...)... Mail Droid which was already there, as a shipping app when I was only starting to work on mine... Enhanced Email which is/was heavily based on stock Email's source code... Did I miss any? Oh, the countless K9 clones.
And then there is me -- on my own.
But -- and @FCasoli is completely right -- the market wants what the market wants. Users don't care about any of the above, they just want features X Y and Z like in mail apps A b and C, and "oh, why doesn't your app do Q, even my phone's built in, basic email app can do this".
So it's kind of turned into playing football (soccer for you USA'ers), in a real tournament, by myself, but against proper 10+1 teams. Somewhat ridiculous, and definitely not what the spectators wanted to see.