Why are Namie Amuros sales so high?

[Baernash]

Answer is pretty simple: her releases are good enough to keep old fans satisfied and bring new fans at the same time.

Namie fitting herself pretty well in different music styles. Her first works (with Monkeys, than solo) were catchy, dance tracks in typical 90' style, than evoluated to something more modern, than she had her "hip-hop" era and than she comes into direction of releasing old style songs in modern version. Now since 2 albums she's doing great with electro-pop in US-style.

orangeakira gave you a good example with Ayu who just started repeat the same formula like "winter single" with some ballads and "summer single" with happy, catchy songs EVERY YEAR. She started to be predictable and all her songs started to sound the same. What she did? Tried experiment with something "modern" but she failed, so she returned to old success formula and released boring album dominated by ballads wrote by her old music composers (those from her debut). It was good and bad for her, because old fans were happy that she's doing something in style which fit to her but on the other hand THEY HEARD IT ALL BEFORE (so no chance to really get new fans). Than another experiments trying to turn her into american PARTY QUEEN (doing some US-pop style tracks etc.) - FAIL. Now she's returning to ballads again but she can't really decide in which direction to go. She's trying to keep all fans satisfied and doing everything at the same time: in one song she's rocker, in other half-naked party girl, in another sweet kawaii girl singing Hello!Project style song (with cute, childish voice), than she's filling her albums with ballads. No constancy at all. Namie by all time keeping good level in what she's doing - she's developing herself as artist, not turning back and not trying to always do things which are popular in music industry.