Why does the decade-old TI-84 Plus still dominate the US market?

The TI-84 Plus graphing calculator has a 15 MHz processor, 24 kilobytes of RAM, and a 96×64 black and white screen. It has an Amazon list price of $190. The Kindle Fire HD has a 1.5 GHz processor, 8GB of memory, and a 1280×800 color screen. Amazon list price: $140. And yet the TI-84 keeps on selling, its hardware largely unchanged from its first introduction in 2004, The Washington Post says it’s the US’ best selling graphing calculator, shifting more than a million units last year and capturing 93 percent of the US market.