ELI5 LED Bulbs - why the blue one warranted a Nobel prize?

What is so special about the blue variety of LED? Why do LEDs still get so hot if they are supposed to be energy efficient? If LEDs were invented in the 1960s, why have they only become popular in the last decade or two? Why did the blue LED warrant a Nobel Prize in Physics?

... We explain like i'm five!

Thank you to the r/explainlikeimfive community as always and in particular the following users whose questions and comments formed the basis of this discussion: laser_lens_4, spud4, jtw24, brierrat, loderman, audigex, chickencereal, agouti & emblem619

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