Adam Smith's wonderfully wise and sadly overlooked other book, The Theory of the Moral Sentiments (1759), tells us why:
We do not, therefore, thoroughly and heartily sympathize with the gratitude of one man towards another, merely because this other has been the cause of his good fortune, unless he has been the cause of it from motives which we entirely go along with. Our heart must adopt the principles of the agent, and go along with all the affections which influenced his conduct...If in the conduct of the benefactor there appears to have been no propriety, how beneficial soever its effects, it does not seem to demand, or necessarily to require, any proportionable recompense.
In other words, since the GOP is not known for "caring about people like me," as the pollsters so disingenuously put it (and a perception the Democrats spend a considerable amount of their time reinforcing), "compassionate conservatism" was playing a game it could not win. No matter how much largesse it spread around, no matter what good it achieved, no matter how many people it may have helped, it would and could never receive a whit of credit for it.
As a battered and bruised neo-con (if neo-conism still exists at all), I can say it was still worth a try, but I must defer to the wisdom of the ages, and the estimable Mr. Smith. The Theory of the Moral Sentiments is the Democrats' playbook, and stealing pages from it is folly, since they can get credit for doing absolutely nothing just by paying lipservice to "caring."
Strangely enough, the playbook for the party of Lincoln, evangelicals and others not bent toward materialist philosophy remains Adam Smith's second tome, An Inquiry into the Nature And Causes of the Wealth of Nations (1776), a far more worldly tract.
"[Americans] still prefer common sense conservatism to the alternative...Common sense conservatives believe that the government that governs least governs best, that government should do only those things individuals cannot do for themselves and do them efficiently," McCain addeth sagely, and hopefully.