The reason is the engineering culture and the promotion system. People expect to switch to new projects, and work towards to a promotion within 2-3 years. This shows in all products, and presumalby also the reason for the deprecation. New people come on the project, want and need to do a major overhaul, and do not have resource or incentives to support the old stuff.
The other half is that most projects are done efficiently by relatively small groups of engineers. I remember that Android Market was still a few people in MTV, with no payment possibilities outside of the US, when Apple already had significant revenue from apps. And that shows everywhere. I'd argue Google is incapable of tackling projects that require more than a few hundred people for more than 2-3 years.
But as a billion dollar company, that does not work. Throwing money at obvious new billion dollar opportunities like Microsoft did with gaming and cloud, and following through over 5 years is not possible for Google.