Why have governments not decided on the final resting place of radioactive waste?

I have no answer but a guess

May be because ,well, if you dump them in space or anywhare on earth it would be bad for the environment

or maybe because they don't know any safe way to dispose them

Or maybe they do not care to find a way and a place to dump them (gov s like USA who didn't even sign the Kyoto agreement)
and expect to have a "Day After Tomorrow" in near future

For a start, you have the issue of it being a political hot potato.  Which voters would you alienate by burying nuclear waste in their back yard?

You cannot send it into space for fear of another Challenger disaster.  Imagine the consequences if an exploding shuttle (or any other rocket for that matter) was carrying large quantities of nuclear waste.  Chernobyl eat your heart out!

Burying it at sea is not an option because of environmental consequences; imagine what Greenpeace would have to say about radioactive whales!

As it stands, there is reatively little nuclear waste around at the moment compared to what there probably will be in a few decades time.  Currently, we obtain a large percent of our electricity by burning fossil fuels.  They won't last forever and we will need a reliable alternative.  Wind, solar, wave, tidal etc probably won't do it, so nuclear is (currently) the best option for generation of large quantities of electricity.  We reprocess much waste, but still there is some pretty persistent stuff which needs to be stored for centuries.  This will be a much bigger problem as time goes by.  

My feeling is that whatever is decided will prove to be extrememly unpopular with a vocal section of society.  While the decision can be postponed, weak governments will.  There is always the hope that when the other guys get in, they can be attacted for whatever they decide.

Another posibility is that they are waiting for new technology to be developed which will provide better solutions than those currently available.

I don't think that there is just one reason, more a combination of a number of things.