Because friction driven dynamos offer advantages over higher powered battery lights (i.e no charging which is frequent with high watt lights and limited light time per charge and expensive batteries)
This light does not offer that significant advantage over a cheap self contained LED blinkie as they can last many days of use on a single battery charge, but does add the disadvantage of more complexity (yes the marketing says it is reliable, but keeping four magnets in plastic containment attached to spokes on wheels aligned to the inducutor coils will be one more thing to deal with for wheel maintenance.)

Al