I've seen it repeated on multiple sites that Planescape Torment sold 400,000 copies, whereas its predecessor Baldur's Gate sold 5 million copies, across various forms (as of 2006). Now that doesn't include digital copies since Torment only became available on GOG in 2009/2010, but consensus seems to be that it was an outstanding game with rather mediocre sales.
So while I love Planescape Torment, I'm not sure it's a winning bet for a big screen adaptation.
Those numbers sound in the ballpark from what I remember reading and you're absolutely right, however remember that:
1. Baldur's Gate had the advantage of being the first big cRPG out of the block. It was the one that revived the genre.
2. The cover of the game featured a rather homely Nameless One in blue and a lot of orange for the background. It wasn't very attractive and a lot of people, rightly or wrongly, base their impulse buys of books and games on the cover art.
3. The setting was very different, one that either appeals strongly to you or makes you roll your eyes. Sigil and the Outer Planes aren't your run-of-the-mill fantasy setting.
4. Regardless of all the above, I'm a Planescape fan and I really wish that it had been carried over to 3.X. In a similar vein, I'd to see the story of the Nameless One on the silver screen, dark ending and all.