The short answer is that the founder is an introverted guy who sort of weathervanes around when it comes to his vision for the product, he had enough prior success to get lots of funding for his project, and there was no one with enough authority to say "no" to his shifting ideas or replace him with a more effective boss.

You see this pattern a lot with successful people. Nassim Nicholas Taleb is a brilliant guy, but his later books are unreadable because he won't consent to have an editor. Your favorite band takes five years to release a follow-up to their breakthrough album. Pundits get a sinecure at a major news outlet on the strength of their insightful thinking and then start producing drivel.

Ev Williams had the misfortune of being given a limitless budget and the freedom to realize his vision. Medium with three developers and a half million dollar budget might have been unstoppable.