It's really hard to get a teacher to change platforms (or textbooks or whatever) when they've got years of expertise and teaching materials built up on their current system. This isn't just laziness: you really are a less effective teacher in classes where you're making a big change like this. (You'd think "but calculators are all the same!" would make that moot, but the trouble is that the kids aren't experienced enough to get that yet, and as an instructor you're suddenly rewriting every single worksheet and hint sheet you have so the instructions match the new system.)

Mind you, I'd love to see the TI monopoly get broken! But this is what you're up against. And even if you have a great product and a teacher might be willing to make the change, they'll be really cautious about the risk that your product won't survive and they'll need to switch back to something else two years later.