Why did LEDs take so long to appear as light bulbs?
It is not possible to produce white light without an efficient blue LED, either using RGB LEDs or a blue LED + yellow phosphor.

The breakthrough was the invention of the high-brightness Gallium-Nitride blue LED by Shuji Nakamura at Nichia
in the early 1990s.

It still took a while to get the overall efficiency up to the level of fluorescent bulbs, and it's only in the last decade that LEDs finally came out on top.