Why on earth? AFAIK, no Habsburg archduke was ever even on the cards. The premier candidates were Bourbons.
Maria Cristina, the Queen-Regent, wanted her brother, the Conte de Trapani to marry Isabella, since she had the reasoning her own marriage with her uncle had been happy enough, why shouldn't a second avuncular marriage be.
England preferred the liberal duque of Seville - Enrique. While France preferred the more effeminate duque of Cadiz. Isabel herself wanted to marry Enrique, however, needs must, and she was married to his older brother instead.
The Carlists talked of marrying her to the Count of Montemolin, but nothing ever came of it.
Other possibilities kicked around were Prince Alexander of the Netherlands, a Saxe-Coburg-Prince (perhaps Franz, younger brother of the king of Portugal and the duchesse de Nemours) and a prince of Parma.
AFAIK, no Habsburg was ever seriously considered.