That used to be an option. However, some changes were made that allow us to only see 10 posts per thread (on a page). I liked the option to see more posts as well. In the overall scheme of things, it's not a big deal but I see what you are saying....
Why can't it be a user option? Sometimes when the threads are very long such as 30,50,100 pages it becomes time consuming reloading each page. I tend to not read them because of this. I would guess that most would leave the default where it is but on the longer threads woulds be nice to increase at my option.
The reason the option was eliminated is because it speeds up the site by making things easier on the server. Having to transfer less data with each individual page query saves a lot of bandwith in the long run seeing as several people will just read the last page of a thread. While it would be nice to see more posts per page, I'll take a few extra page loads over the site running painfully slow like some of the *ahem* "other" forums.
Thanks for the change and quick decision making. Looks like I can change it to 20 max, right? When I changed it I had and error about the URL jump. I had to put something in there so I could save it.