One of the advantages that the “incumbent” party has under the norms of American politics is getting to have your convention second.  (Depending upon how you count, this tradition has been followed either since 1936 or since 1956 — before 1936 Democrats tended to, but did not always go second).  When, as in this year, there is only a three-day gap between the two  conventions and the nominee of the out-party waits until the last second to announce his pick, that gives the nominee of the incumbent party a chance to finalize her pick without much media attention.