The decision was made years ago: No pipeline.
Not announcing the decision stops the Koch bros and the Keystone corp from starting their appeal. Its like an administrative filibuster.
There is already a pipeline that runs to St Louis, the only reason to build the second pipeline is to sell the sludge to China. Having that option available will allow the price to be jacked up when the sludge is sold to the US market as it will fetch the international price which is a lot higher than the refiners currently pay in St Louis.
There is absolutely no reason for the US to OK a pipeline that will increase the cost of supply to the US market. The only reason the GOP backs the pipeline is that the Koch bros stand to make $100 billion from the increase in the value of their shale tar sands.
It is a purely tactical decision because nobody outside the GOP wants the pipeline built. Everyone who wants the pipeline will vote GOP in November whatever the decision. Obama could make a short term political gain by announcing that there will be no pipeline but that would allow the appeals to start. Better for the country to wait until there have been some GOP deaths on the SCOTUS.