They were strong in the 1800s because the empire was mostly the crownlands of the Habsburg family inherited from their famous marriage diplomacy. It began from austrian archduke Maximilian I when he married Mary of Burgundy thus secured him the lands of Burgundy together with Austria. His son, Phillip the handsome later married Joanna the mad, daughter from Isabella I of Castile and Ferdinand of Aragon. With these, their son Chares I or V gained lands not only in Spain but also in Burgundy, Austria, Milan, Sicily with Bohemia and Hungary together when the hungarian king Louis II died during the battle against the ottomans in which the Habsburgs were his relatives.
Even though the Habsburgs had inherited many lands, most of them were not united under the same capital and each one has their own law and government seperated from one another due to geography, language and distance. It became an enourmous difficulty to govern all these lands together under a single monarch and Emperor Charles V had to almost constantly fought wars everywhere to keep his lands intact from the protestant rebellions to the french invasion together with the ottoman siege. It was overhelming for him alone therefore he later seperated his empire between his son, Phillip II of Spain and his brother, Ferdinand of Austria so each of them could focus more on their domain but remain allies. The Habsburgs maintained their marriage only within their relatives resulted in the last spanish king, Charles II heirless. After the loss of Spain to the french bourbons, the Habsburgs began to focus more on Austria and the rise of Prussia changed austrian politics.
Hope this helps!
xx,
Myara
