Make change for the better. Especially design. Case in point- Adobe Photoshop CS4 vs MS Word 2007. Both utilize a set of tools to perform functions of the main program, giving users the ability to alter/edit/change/etc. the content. But each program (even the suite it is part of) goes about displaying the tools differently. Upfront I'll tell you I prefer Adobe's display (or UI for User Interface) over MS's display.
Pics (otherwise it doesn't exist):
So why is Photoshop's UI better? Because I'd venture a guess that the majority of content created on computers is for 8.5x11 paper, usually in the portrait format. Users of Photoshop, however, are not using that format. Most use Photoshop for photos, which are mostly 3x5, 4x6, or some other horizontal layout. Photoshop menus can be "undocked" meaning users can move them where they need to.
In Word, the menus are "Gorilla Glue'd" to the top, and users are forced to use Word this format. I don't know if many users use the 2-up "Reading Layout" for composition, but I'm willing to guess we've all accepted the "Normal" or "Page Layout" view as standard- one sheet at a time.
With the proliferation and acceptance of Widescreen monitors (over the 4:3 ratio. We've always been "widescreen"), it makes sense to provide toolbars and the like similar to Photoshop- along the vertical sides of the screen. It preserves the standard upright content form we're used to and have standardized on. If we ever switch to square paper, or landscape everything, then we'll need horizontal menus.