I wouldn't say NPR "leans left", rather that they "lean establishment". NPR has no sympathy at all for socialists, third party candidates, most protest movements, etc. Republicans just have too much political diversity and churn in their base in the last few decades to be anywhere near as uniform and cohesive a bloc and so the establishment usually appears at least superficially Democrat-biased.

Note: Local NPR programs are a lot better than national programs, IMO. There are two available NPR stations in my area, and they're really not similar at all except for a small overlap in programming.