In ten minutes the writer needs to introduce the main characters, establish the genre, let the reader or viewer know generally what the story is about, and what is at stake. Everybody's reading, it's sad to say, for a reason not to take on something. Everybody over schedules and has more writers than they can handle and more projects than they should handle. It's really tough on the writer because they have to present a reason why this is different from all other screenplays and do it on the first few pages.
Time is the biggest reason why scripts are given a ten page limit.
Agents, Producers, Readers, Studio Execs are pressed for time. There are so many scripts and so little time to read them. Unless they are certain they are in for a good read, no one is willing to devote the 90 to 120 minutes it takes to read a screenplay only to end up passing on it. I want to like what I read. People think that you can't wait to send back the rejection notes.