What is the difference between a feasibility study and a thesis?

A thesis widens/broadens human knowledge by going into previously unexplored areas, e.g., by proving or disproving a theory which nobody has proven/disproven before. Or, you might extend present knowledge into new areas, e.g., if nobody had asked the question before, you might seek to prove the speed with which a body fell in a vacuum, rather than in normal air.

A feasibility study is an exercise to find out the extent to which a project can be implemented, e.g., if I built a car park at a given location, what kind of financial returns can I expect? You'd have to forecast demand, supply, competitors, etc.