I've been very fortunate to meet and spend real time with the founders of many companies across a diverse spectrum of industries. I'm starting to recognize a new pattern - almost none of them can quickly articulate what they do.
It's not for lack of intelligence - they're often the brightest people. It's not lack of communication skills or charisma - many are incredibly articulate, often great public speakers. And, it's not for lack of practice - a founder spends a surprising percent of her time telling their story to prospective customers, to new employees, to investors, to anyone who will listen.
Here's my hypothesis about why it's so hard: founders, by their very nature, are different. They see the world differently. They see customer problems and root causes of those problems differently than others. They have a unique vision of a future state. And, they got to this place through a different path than almost anyone they'll speak to.
For lack of a better analogy, founders (and innovators, more generally) are like mutated genes. Most will fail. But, in the future, for the ones who succeed, they'll become the new standard, their new way will become the "why wasn't it always this way". And, like mutated genes, they stand-out. They're different.
Why does this make it hard for them to tell their story? Because what makes the story work for them isn't what makes it work for the rest of us. They often can't empathize with other ways of thinking about their industry or customer problems. They often don't understand why other people don't "get" what's hard about what they've built, why it matters.
Their lack of ability to tell their own story has a compounding effect when they send other people out to tell the story. A newly hired VP of Sales, for example, will often be stuck between a founder, for whom their solution is an obviously better alternative than what their customers do today and a market of prospective customers (and new salespeople) who just don't get it. Arguably, this is why there is so much emphasis on and growing evidence to support, especially among lean startup supporters, having founders make the first few sales themselves.
Another challenge for the founder is getting useful criticism to improve their story. While there are people everyone offering their opinions on what's wrong, why it won't work, how the story doesn't make sense, most of those are the wrong people. For the same reason that you can't build great products by (uncritically) asking customers what you should build next, it doesn't work very well to ask people who aren't likely to "get it" how to tell the story better.
What can founders do about this? Great advisors can help. Fellow founders can help. Testing in the real marketplace can help. But, more than anything else, laser-like focus on simplifying through an iterative process is probably the fastest way to telling a powerful story that resonates with the right people.