Thar coal is of lignite variety containing high ash and moisture content. To top it, it is located in a far off area.
Transporting coal by 10 miles double it's cost. This is usually overcome by setting up coal fired plant near pitheads but in your case, coal reserves are located in arid areas.This lead to an additional problem of lack of coolant water supplies making set-up of thermal plant unfeasible.
Probably it has nothing to do with politics.
The major thing stopping exploitation of thar coal is absence of HT transmission lines to connect prospective power plants with the national grid. The rest of technical and financial challenges have been studied in bankable feasibility studies( read above) and found not to be a problem at all. Hard data says it's viable.
So what is the delay is securing the financing? Because the "feasibility" may not be there?
Same that is with any multi billion dollar project, it takes time, plus the government needs to lay HT transmission line to connect prospective plants to national grid. It requires massive scale development and would require at the very least 10 years to realise.