Spain seems to be conspicuously absent from historical and modern-day discussion about Europe. They weren't major players in either World War, and discussion about the EU today rarely mentions them, in my experience. What are some reasons for this?

Something that is very particular in that regard is the contemporary European 20th century experience was something most Spaniards experienced in the 19th Century!

While many European Empire's had to deal with rebuilding from a significant conflict and losing their possessions abroad in 1945 and beyond, the Spanish had to do so from 1815 onward. The invasions by Napoleon annihilated the Spanish economy and capability to preserve their significant if faltering Empire abroad. Spain was considered a big player at the Congress of Vienna but poor diplomatic performance and an ever weaker hand dealt by circumstances ensured they could not levy any influence on a continental scale again for Europe.

Spain afterwards was then embroiled in a circumstance very foreign to most European Empire's until the 1950's in large scale colonial conflicts against ideologically and nationally organized guerrillas. The Spanish colonies had organized on a much more autonomous level during Napoleon's invasions and had ideological and economic reasons to reject control by Madrid now.

Many of the ruling class in the Colonies opposed Liberal policies imposed by the Napoleonic invasion and preserved by a weakened Bourbon monarchy which threatened to destabilize their positions of privilege and power gained when Spain itself was occupied.
The Spanish economy required rebuilding and therefore would place a heavier economic and financial burden in the form of taxes on the colonies.
The Government in Madrid had failed in providing capable defenses for the Colonies during the Napoleonic wars against stronger and now dominant European powers like the British, who unsuccessfully invaded places like Argentina in 1807. They were repulsed often by entirely local defenses and counterattacks.

All these factors lead the local ruling classes which often supported the Spanish prior to be much more resistant to their rule and often meant the Spanish had to try restoring rule in the colonies by force.

These conflicts both on and off dominated the national debate and foundation of Spain from 1815 to 1833 with a severe economic and cultural implications for the Spanish. The forcible severing of the Peninsula and many of it's colonies left a pride and hard grip on the few that remained like Cuba and the Philippines. It would take me several hours to go into proper detail about the underlying causes and effects or battles for each Latin American state's withdrawing from the Empire. But it was universally not on amicable terms between the Spanish and the former colonies.

While the Spanish Empire collapsed, the maritime trade trickled to the Americas and Philippines, and Spain's military struggled to keep their colonies, with Mexico getting its independence in 1821. The customary overseas revenue to the metropolis was at a historic low, the royal coffers were empty. Financing (solvency) and recruitment to the military became an overriding concern for the Spanish Crown, with the governments under King Ferdinand VII failing to provide new solutions and stability.

Against a backdrop of on-off bankruptcy and solvency issues, towards the end of his life, Ferdinand VII promulgated the Pragmatic Sanction giving hopes for a liberal rule. Ferdinand VII of Spain had no male descendant, but two daughters, Isabella (later known as Isabella II of Spain) and Luisa Fernanda. So he promulgated the above "Pragmatic Sanction", to allow Isabella to become Queen after his death, returning to traditional rules of Spanish succession. Without the above Pragmática Sanción, Carlos de Borbón, the king's brother, would have normally become king. He and his followers, such as Secretary of Justice Francisco Tadeo Calomarde, pressed Ferdinand to change his mind. But the agonizing Ferdinand kept his decision and when he died on 29 September 1833, Isabella became the legitimate queen. As she was only a child, a regent was needed, so her mother Queen Consort Maria Christina was appointed.

With conditions like that established, Spain wandered into a civil war of unseen character with the Carlist war. 2 more would break out over the course of the next 40 years and would prevent Spain form ever reaching the national and ideological unity to allow itself to move forward like the French did following the Napoleonic wars. The 19th century therefore was always one of political unrest and economic decline for the Spanish. They couldn't recover from the invasion by Napoleon because it's residual effects continued to torment individual governments in Spain either financially or politically.

Spain entered the 20th century on the backpedal of a war against the United States who managed to utterly annihilate Spain's armed forces and severed their treasured remaining possessions in the Philippines and Cuba. This defeat encouraged the growth of a new intellectual and political grouping of people called famously the Generation of '98.

The Generation of '98 intellectuals objected to the meticulously organized structure of the Restoration system of government and the corruption that it fostered. After Spain's bloody and decisive defeat in the Spanish-American War, which resulted in thousands of dead Spaniards and the loss of all of Spain's remaining colonies in the Americas and the Pacific, these writers were prompted to voice their criticism. They agreed on the urgency of finding a means, in areas of thought and activity separate from politics, of rescuing Spain from its catatonic state.

The writers, poets and playwrights of this generation maintained a strong intellectual unity, opposed the Restoration of the monarchy in Spain, revived Spanish literary myths, and broke with classical schemes of literary genres. They brought back traditional and lost words and always alluded to the old kingdom of Castile, with many supporting the idea of Spanish Regionalism.

This regionalism and local thinking left the repeatedly weakened and poor Spanish state to be extremely introverted in whatever format it was until the Spanish civil war.

Due to King Alfonso XIII being related to all major Monarchs who's nations fought in the great war and the relative poverty of his state Spain remained predominantly neutral for the First World War. Spain was notorious however for how many arms it exported in the war due to it's byzantine copyright laws that allowed ramshackle and robust arms production of foreign design to pop up and this found a clear market in the armies of WW1.

The end of the 1st world war for Spain was disastrous to it's black market of arms, but even more so would be the outbreak of the incidentally named Spanish flue. Alfonso became gravely ill during the 1918 flu pandemic. Spain was neutral and thus under no wartime censorship restrictions, so his illness and subsequent recovery were reported to the world, while flu outbreaks in the belligerent countries were concealed. This gave the misleading impression that Spain was the most-affected area and led to the pandemic being dubbed "the Spanish Flu."

Spain thereafter entered the Rif War which was a conflict in northern Morroco which was deeply unpopular with many Spaniards, Alfonso took a lot of blame for failures during the war and whilst it was successful with a French bailout by force it is considered a progenitor of contemporary European decolonization conflicts.

Spain's poverty and political unrest continued to grapple it even after Alfonso abdicated in favor of a new Republic in 1931, the 2nd Spanish Republic was considered an international novelty with no real weight which was compounded by political division and an increasingly large force of reactionaries in Spain.

By 1936 the Spanish Republic found itself hinging on an election where war was promised if the Government's weakness continued. A new Government formed and attempted to move away powerful reactionaries in the military and bureaucracy to away from the mainland, this partially served to prevent a fascist coup. But the Falange (Phalanx) party and the military still managed a partial coup that lead to the infamous Spanish Civil War. I cannot elaborate too greatly on that due to the long character of this already. But it was won by the Fascists with the aid of Mussolini and Adolf Hitler.

Spain's low key in world war 2 was similar to the first. It was recovering from a devastating and divisive conflict that left Spain with little need or will to fight in a worldwide conflict. Dictator Francisco Franco's aloofness allowed his regime to be one of the very view Fascist states to survive the 2nd world war. This came at a cost however as the other European states refused to engage Franco on a diplomatic or allegiance level. Spain until his death remained isolated from European or trans-Atlantic projects like NATO or the EDC. It handed over it's few remaining overseas possessions without much fanfare or controversy bar West Sahara, which is still occupied disputably by Morocco today.

Spain is seldom mentioned outside of modern day discussions in the context of things like the World Wars or the European Union because this exclusion was of the Spanish leaders own design at the time. Spain had unique developments outside of the general European experience because they experienced it well before most of Europe did. When the French bloodily fought in Indochina and Algeria, Spain already had in the Rif War or the Latin American Wars of Independence.

Anyhow, that tries to put roughly 200 years of historical context together in a legible way. Hopefully that answers your question.