Speaking of human classification...why aren't there any subspecies used for man, the most widespread species of mammal on earth?
Because there are no obvious cutoff points. All geographic variation in humans is clinal, gradual. What is more, every gene has its own geographic variation; few if any correspond to the variation of the 6 genes for skin color or the 3 for eye color (I hope I'm remembering the numbers right).
Even before all this genetics stuff was known, remember that anthropologists never had a consensus on how many races there were. In the late 19th and early 20th century especially, the proposed numbers ranged from about three to no less than sixty-six.
And that's before we get to the social dimension of it all! Even today, the same person can easily be "black" in the US, "coloured" in South Africa, and "white" in Brazil.
In short: humans are about as promiscuous as bonobos, and have always been. The only reproductively isolated, or anywhere near seriously isolated, human populations have been the Easter Islanders for something like 400 years and the Tasmanians for maybe a few thousand; everyone else participates in the chaos of crisscrossing clinal variation. There's simply no point in trying to recognize subspecies there.