I almost failed geometry in high school. And I blame my lame pool-shooting skills on the fact that I almost failed geometry. You see, pool is all about angles and lines. And there's your answer!

Different types of bikes have different angles. Time trial bikes have a different degree rake on the fork and seat tube than a road bike. And a cyclocross bike is even more different. The angles will affect handling (among other things). For example, a cyclocross bike isn't set up to corner at high-speed like a road bike. The designs accomodate the demands of the sport.