The fact that some Americans supported slavery in the eighteenth century is not at all remarkable. Most of the world agreed with them. What is remarkable is that many of them sought to abolish slavery in the new republic.
In a slave economy, slave owners seek technological innovations that make slave labor more productive. But they also place inefficient and artificial limits on innovations that might change the established social order.
By flooding the market with cheap credit, Alan Greenspan pushed interest rates (including mortgage rates) down to artificially low levels. This caused the bubble in house prices and misallocated too many real resources to the housing sector.