The "browser" half of the CAB forum is happy to accept new CAs that meet their standards. Non-profits and disruptive startups are finding that difficult, as they should—it's hard to run a CA safely. It's not hard like curing cancer is hard, or even like heart surgery is hard. It's hard like avoiding iatrogenic infection is hard: it requires constant and diligent adherence to rules in the presence of no visible threat.
LE are a wonderful start, and they are succeeding in part because they're moving slowly. I expect them to go further, but only with full understanding of the obligations they take on by doing so. They're very, very right to be conservative in their operational practice, because they'll be allowed no more slack than anybody else, and maybe less.
Could something like LE have started ten years ago? Maybe, but nobody did. CAcert.org was a lovely dream, but didn't (and couldn't) hold to those standards of diligent practice.