Private trackers (at least, the "good ones") take time and effort to get into and stay in... most people just want convenience.
Is it much less expensive?
As GabeN has stated: piracy is a service problem, not a pricing problem.
I will provide a concrete example. My buddy got into anime and was raving about one specific title so I checked Hulu for it, but Hulu, for some unfathomable reason starts that anime at season 4.. If I want to legally Stream season one, I would need to try the Sony owned anime thing, which I refuse to do for reasons not related to streaming wars. I ended up buying a dvd ( cheap and good enough quality for me ).
And pirates... have everything and, unless you are looking for newest releases, is of superior quality.
Movies, apple TV, netflix, Amazon, Disney, hbo, sports PPV