I think
davem's got it spot on (how many times have I thought that before?). The films
as they are are already laced with a hefty order of melancholy, in the eyes of movie-goers. The 'normal folk' surely could not handle any more sadness - so assume the filmmakers.
But like many others on the site, I would have liked to have seen a darker tone to the movies. That might sound strange, what with the plot and all (so don't point that out
Saucepan 
) but Jackson only punched half-heartedly when it came to the tragedy and dejection.