Idiom reflect life around us, and the lives of our ancestors, who would make comparisons with the animals they saw, their bodies, clothes, etc, as part of the immediate world, and may idioms are very old. 'Mad as a March hare' may not seem an obvious idiom to those, like me, who have lived an urban life and rarely see hares at any time of the year, but in older times, more people lived on the land.
There are figures given for each category in our list here:
Categories of English Idioms - UsingEnglish.com
Body (425) and animals (319) are way ahead- the next is money (95).
This is not a scientific measure as the list is far from complete and there are often questions about which category to use, especially where there are two or more possibilities- for instance, 'on cloud 9', where we have to choose between numbers and the weather. (At the moment, we can only choose one category) However the fact that two groups do stand out by such a wide margin does suggest that they are probably the most common. As physical beings in a world with animals around us, who behave in set ways, it seems logical that we have so many of these.
I wouldn't make any claims for the value of the data at the bottom of the list- the figures are simply too small to suggest any sort of accuracy or meaning IMO.
