The same rationale that drives all tech companies to change their products: featureitis. If you are maintaining the status quo then you are not evolving and that lack of new features could cost you customers/eyeballs. The problem with featureitis is when it changes what works and adds stuff that doesn’t…. And based on the reaction that Facebook has gotten in the social network that they themselves helped pioneer… Well, much of what they added doesn’t appear to be well received.
Finally, I’d say the real-time ticker is absolutely a direct reaction to Twitter… Whether it is a well-done and successful reaction remains to be seen.