What are some reasons for decline of georgism? I understand that Georgism was a major economic idea in the late 19th century.  The idea of a simple land-value tax and basic income derived from an equitable distribution of natural resource exploitation seem to be great ideas solving many of the challenges faced in other economic systems.  Also, Progress and Poverty was one of the most popular books on economics in the late 19th century.

Why has this seemingly great concept lost popularity?  My speculations is that it either does not benefit the ruling elites (so, politics) or that, more plausibly, the idea of basic income derived from natural resources was calculated to be unsustainable amid the population growth of the turn of the century.
Warren Samuels addresses this issue in his article "Why the Georgist Movement Has Not Succeeded: A Speculative Memorandum."

In summary he argues first that Georgism did not succeed because of the conflation of the ideas of income and productivity, leading people to view a tax on land as a tax on productivity even though the tax is meant to equate income with productivity as referenced in the question. This idea, he says, was reinforced by self-interested beliefs of land owners as well as traders in other markets (he names equities) including preference for passing property through inheritance and viewing the proposed tax system as a threat to income through speculation (such as through the stock market), since it would eliminate speculative gains in land prices. The rise of mortgages and home-ownership increased the number of people with these self-interests.

Second, Samuels addresses the fact that George was viewed by many, including mainstream economists, as a radical and admits that relative to the status quo of changes in tax policy being incremental, the proposed single tax system is radical. The Bolshevik revolution in the 1910's and subsequent red scare caused people to distance themselves from ideas that could be viewed as radical leftist. 

Samuels continues on to discuss possible internal reasons, such as lack of leadership amongst George' successors, but notes that he is less confident in discussing these factors. 

In addition to this article, four responses and Samuel's response to the respondents can be found in the Jul 2003 issue of American Journal of Economics and Sociology.