You asked why has it taken LED's so long to be used as street lights. Led's seem to have several limitations. One is that they can't be made larger apparently. Each led is about the size of a small christmas tree light. To get a brighter light output you have to put a lot of them together and/or increase their brightness. You can only get so much light out of one little christmas tree light, so you have to put lots of them together. Now maybe you can see the problem. Getting a streetlight's worth of light output from christmas tree type lamps.