This is due to the fact that within an enterprise the page rank algorithm that Google search is based on would be less vulnerable to spoofing and disruptive search engine optimization. Links between pages within the enterprise that are not legitimate or serve only to puff up an individual's particular contributions will be able to be quickly spotted and purged, and offenders can be disciplined.
Also, the various apps that Google has rolled out to the public would make a compelling package for an enterprise if packaged within a Google hardware appliance. I would see the package as follows:
-intranet search (i.e. Google's standard search functionality)
-alerts--employees receive alerts when content is posted to their company intranet
-blogger--employees can use a blog as a method of communicating to co-workers vs email
-calendar, docs, and spreadsheets--obvious replacement for Microsoft products
-gmail as in-house mail server--obvious replacement for Exchange
-wiki-Google will do something with the JotSpot technology they've got--obvious replacement for Notes and Sharepoint