The principal reason, invariably, most "successful" giant companies rather quickly become also-rans, or just amorphous blobs on the competitive landscape, is their failure to re-tool in anything like a fundamental way. In fact, the worse things get, typically, the more they dig in their heels and defend yesterday's turf.
Source: The Real Stunner!: http://www.tompeters.com/blogs/main/010476.php
Contributed by: ~C4Chaos