I think the nature and demise of Spain’s empire is key. It was a massive, globe spanning empire with control of vast silver and gold - almost none of which remained in Spain. The Spanish Empire was stuck on a debt treadmill whereby other European powers hoovered up all that gold. Merchant cities in Italy and early capitalists in the Low Countries and England were huge beneficiaries of all that American gold and silver the Spaniards stole.

That meant the Empire was never capable of doing productive capital reinvestment back into the metropole. It never really got to industrialize and build the productive capacity to challenge the rising capitalist empires. By the end of the 19th century, it was hopelessly outmatched militarily, economically, and industrially by those capitalist empires and didn’t get to meaningfully participate in their imperial competition.

Spain didn’t develop a large industrial proletariat and mostly remained an underdeveloped agricultural peasant economy even into the 19th century. When the revolution failed, fascist Spain was backwards compared to the big European powers and sat on the geographic abd political periphery of the imperial core.

Spain has just never had the same skin in the imperial capitalist game as the major White Empires.