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

In the ruthlessly competitive world of technology, where companies rush the latest gadget to market and slash prices to stay competitive, the TI-84 Plus is an anomaly. Texas Instruments released the graphing calculator in 2004, and continues to sell it today. The base model still has 480 kilobytes of ROM and 24 kilobytes of RAM. Its black-and-white screen remains 96×64 pixels. For 10 years its MSRP has been $150, but depending on the retailer, today it generally sells for between $90 and $120. The only changes have come in software updates. Amazon calls the TI-84 Plus a No. 1 best-seller. Texas Instruments says that this year the TI-84 Plus C Silver Edition has become its best-selling calculator, and that the TI-84 is its most popular family of calculators. The TI-84 Plus C Silver Edition is slightly more expensive than the base model, has a color screen, rechargeable battery and significantly more memory. Even with a 320×240 pixel screen, 128 kilobytes RAM and 4 megabytes ROM, overall the TI-84 line of calculators appears unnecessarily expensive given the components.