Geoplatform.gov is actually built on ArcGIS Online which is running on-premises in the GSA hosting environment. Geoplatform.gov currently has a small set of publishers who 'curate' the content that is visible, with a focus on web services.
When Geospatial One-Stop retired, it was integrated into the Data.gov website as http://geo.data.gov. Geo.data.gov IS built on the open source Geoportal Server (http://esriurl.com/
There were close to 650,000 items in Geospatial One-Stop, many of which were from state/local government or from academia and do not meet the criteria (http://www.data.gov/
To provide access to that full set (by now grown to about 950,000 geospatial resources), the search from the Geoplatform.gov site was included.
There are really two programs (Geoplatform and Data.gov) that are looking at how to best implement their objectives/mandates: serve nationally significant geospatial data assets from and to the geospatial community at large) vs increase public access to high value, machine readable datasets generated by the Executive Branch of the Federal Government (http://www.data.gov/about).
What you see now is (hopefully) a transition from the Geospatial One-Stop period into the new open government data period where these two objectives/mandates unite...