Here's my ingame explanation:
All complicated life is imprinted by the mark of the divine. It was the gods interaction with the work of the primordials that resulted in life more complex than simply that of undifferentiated flesh, energy, or oozes.
As such it is extremely simple for clerics and divine casters to manipulate or to manipulate through mundane means, but arcane magic itself which essentially works within the most primitive strata of magical reality finds it difficult to work through the matrix of the divine that surrounds all life.
There are work arounds for this. Dead flesh is easier to work with since it is simply flesh and the divine matrix has already started to break down. The presence of that matrix on the flesh, however, also gives Divine users special privelege over it as well.
Bardic magic's reliance on the magic of song as a matrix makes it the most efficient arcane means of accessing the life matrix due to its combination of mundane materials, increased influence of 'natural' or primoridal mysticism, and the affect of that particular style of magic in increasing the receptivity of the target.