The question asks why the distribution of coil, alpha-helix and beta-strand is 60:30:10 rather than 33:33:33. The answer is: 

>“Why not”

I only answer to state the obvious because this was voted a good question by two other people who would seem to share the mistaken scientific logic of the question.

What earthly reason is there to expect that three types of structure (or in the case of coils, lack of structure*) should be present *in equal amounts* in proteins? It is like expecting that the percentage intron, exon and ‘junk’ DNA should be equal, or that the percentage of fuel reserves stored as glycogen and fat should be the same. Yes, they belong to the same category, but they are sufficiently different in each case for one not to be surprised if they are not required in equal amounts.

Contrast this with situations where the prior expectation might well be for equal usage, and the deviation from this could be regarded as bias and an explanation worth asking for:

 - The different proportion of the 20 amino acids in proteins (although a chemist would not expect equal proportions)
 - The different usage of synonymous codons of the genetic code in various species and mRNAs
 - The different usage of the similar molecules with a high phosphoryl group transfer potential: ATP, GTP, UTP and CTP (often loosely termed ‘high-energy’)

***Footnote: A coil is not a secondary structure**

As is stated in the Wikepedia entry for [Protein Secondary Structure](https://en.wikipedia.org/wiki/Protein_secondary_structure):
> The random coil is not a true secondary structure, but is the class of conformations that indicate an absence of regular secondary structure.

It might be mentioned in regard to protein structure that there are smaller [three-dimensional motifs](http://motif.gla.ac.uk/motif/index.html) that the analysis quoted does not consider. These do not occur in equal proportions either, to nobody's great surprise.