Why aren't 'exons' named 'introns'?
The terms intron and exon were coined by Walter Gilbert in a renowned 'News and Views' article, Why Genes in Pieces, published in the journal Nature in 1978.

Introns are the intragenic regions and exons are the regions which are expressed.

This is the relevant passage in full:


  The notion of the cistron, the genetic unit of function that one thought corresponded to a polypeptide chain, now must be replaced by that of a transcription unit containing regions which will be lost from the mature messenger - which I suggest we call introns (for intragenic regions) - alternating with regions which will be expressed - exons. The gene is a mosaic: expressed sequences held in a matrix of silent DNA, an intronic matrix.