Smell has no simple counterpart to the three basic color sensors of the retina. The optic nerve that carries the retinal output to the brain is a thick cable with a million signal paths. Each retinal receptor remains traceable as its signal traverses that living cable. Even within the brain itself, the output of a small retinal patch can be traced as it is relayed again and again, until finally layers with more complex signal processing are reached. The space of odors is home to no such simple mapping.
Diversity counts more than any defining set of basic stimuli.
And the neural pathways found in the olfactory bulb are visibly tangled and loopy. Details are not plain as yet, but we can already scent best-bet associations made by a complex neural network. This most ancient sense seems to be processed at once by such a network, one not encountered in the hardwired visual system until much higher, more intricate levels of the brain are reached.