Tax Liens and Inflation
Not so long ago, rents never kept up with rising costs of property ownership, so even after all the tax benefits were factored in, investors were committed to a net negative cash flow. This was rationalized with assumption that inflation would raise the value of the property to a level at which monthly payments would be recovered with a profit to boot.
The deflationary environment which began in 1981 burst a lot of speculative bubbles, but even now tax liens investors commit themselves to ownership of property knowing that there will be negative cash flow and less potential for appreciation.