In 1962, Nick Holonyak, Jr. of the General Electric company patented the first practical LED and by the late 1960s they were in common use.
Why did it take 40 years for LED Light Bulbs? Mostly cost, just like with anything. Although it wasn't until the mid-2000s that LED flat panel TVs came into the consumer price range, for example, they were debuted in 1977.
Here's a graphical history of LEDs. Let the age of better lighting begin (and don't buy CFLs)!
Via: Go Green LED Bulbs