1. Low value of products to users
Blackboard, WebCT, and other similar 'Learning Management Systems' are products of a standard response to new technology in an established area. Essentially attempting to copy the existing way of doing things into the new mode and thus make them more efficient. Moving from the existing systems to internet systems might reduce costs by half or so, if you have a really good system, but it isn't 'disruptive'. This isn't spreadsheets or email. It isn't 1000 times better than the alternative.
2. Institutional inertia
Mentioned by others already, you're going to have a hard time selling to institutions. You're competing against a bunch of giant vendors with existing relationships, and FOSS solutions which are backed by and used by multiple universities. My university struck a committee of a dozen people with little to no knowledge of eLearning and spent probably $50,000 in employee time before deciding to maybe try moodle, or just stick with WebCT.
If you've got an idea for how to meet the real needs of educational users several times better than blackboard go for it. Personally I'd love to see something blow the existing junk out of the water, but think carefully about what people actually need and how you can serve that need don't just make a better blackboard.