[wtallis]

They've set up a great lock-in system. They've bought off all the schools and teachers to get them to require students to use a specific TI model, and then they get the schools to buy textbooks that are written to use the TI calculators, with examples that give keystroke-by-keystroke instructions, and sample programs written in that model's dialect of BASIC. Sometimes they even get the standardized tests to require a specific calculator.

It's a great deal for lazy teachers: not only do they not have to teach their students how to perform the calculation, they don't even have to teach the student how to use their calculator to do it. If a student wants to use a different model, they're on their own. They'll have to learn the TI BASIC anyways, because the textbook doesn't even try to do a decent job of describing an algorithm in any other notation.

In most cases, breaking TI's monopoly would be as difficult as, and possibly require, busting a teachers' union.

[sliverstorm]

I've heard the other reason is their math libraries. I don't know a whole lot about the calculator industry, but I've been told the number of companies with good embedded math libraries is very small, and the cost of developing one is prohibitive.

This adds big barriers to entry. You can't write your own libraries, and you can't use more powerful hardware to compensate either- TI's calculators cost them very little (the popular TI-83 uses a CPU designed in 1976), and will shove you right out of the market with their huge profit margin.