I believe the answers are environment and geography.
The main reason why the Founding Fathers made sure to reserve some powers for the states is that they wanted to ensure that the federal government wouldn't become too powerful, since they were afraid of tyrannical governments of any kind.
B. Allowing newly added territories to the United States
decide for themselves the slavery issue
Popular sovereignty emerged in 19th century America as a
compromise strategy for determining whether a Western territory would permit or
prohibit slavery. It argued that in a democracy, residents of a territory, and
not the federal government, should be allowed to decide on slavery within their
borders
World War I--in the end WWI is connected to the annexation as the Serbian attack on Archduke Franz Ferdinand was a response to the annexation.
Following the Austrian annexation of Bosnia and Herzegovina, Serbian nationalists called the Black Hand planned an attack on the Archduke Franz Ferdinand's visit to Bosnia. This assassination, triggered the events which led directly to the outbreak of World War I.
