In the "inverted pyramid" the most important information (which should come first) is represented by the base, which is the biggest part of the pyramid and holds up the rest of the pyramid. In a sense, it is the foundation, so you have to "get it right".
The analogy is "base = big = foundational = important"
Personally I think that's confusing, because you just as easily say the tip of the pyramid should represent the most important information, which should be conveyed concisely and without extraneous detail or background.
In that case the analogy would be, "tip = concise = main point = important"