Maybe the explanation is in the traditional government practice of rice-bowling. Here is what the article says,
Karen Evans, administrator of OMB's Office of Electronic Government and Information Technology, said it is one thing for agencies to offer a service, but it is another for them to actually shut down their similar service and use the governmentwide solution. [my italics, of course]
That is why there is an Army Single Face to Industry website and FedBizOpps, a USAJobs site and the Army CPOL website (the Air Force has one and the Navy, too).
Of course, what about all the legacy systems that are procurement-related that can barely even talk to themselves.
The Coast Guard's contract writing system (see the article or read the report, see page 8) that doesn't talk to the federal government's new repository of procurement-related data (FDPS-NG) nor to other DHS contracting systems. The report says
Currently, however, DHS has several different contract writing systems that do not automatically interface with its Federal Procurement Data Systems - Next Generation (FPDS-NG) – a government-wide procurement reporting system accessible by the public. Some of the systems may need to be replaced. Additionally, not all DHS procurements are entered into FPDS-NG. For example, grants, mission assignments, and purchase card data may not be entered into FPDS-NG, resulting in an understatement of DHS’s procurement activities.
Wow! Why can't we just get along?