The short answer is that they weren't always really trying to. There are several reasons for this.

First, there were, over the course of the period between 1212 and 1482, when the final war between Castile and Granada started, a number of key political treaties in place between the Christian and Muslim kingdoms. Castile and Aragon were definitely the primary powers in the peninsula, both militarily and economically, but each kingdom had trade value, and war prevented that. Thus, it was often in their best interests to trade across religious lines.

Second, and relatedly, while the Nasrid kingdom (of Granada) and Marinid kingdom (of Morocco) and the various smaller Muslim kingdoms of the Islamic west no longer had the unity and power of the Almohads, they were still had significant military power they could bring to bear, forcing Christian leaders to be cautious. Muslim power in Iberia was not broken in 1212.

Third, and perhaps underlying all of the above, is the fact that Reconquista fervor was not constant, nor an overriding concern. Yes, the conquest of Muslim territories was often coupled with crusading rhetoric, and was sometimes seen as an important task on God's behalf. However, zeal for this endeavor ebbed and flowed. It was not the underlying purpose of the Iberian Christian kingdoms, nor did all kings and nobles necessarily want to make it a priority.

Lastly, there were lots of distractions. The centuries between the fall of Cordoba and Seville (1236 and 1248 respectively), after which time there was a relative lull in conquest, and the final war between Granada and Castile saw a number of wars between the Christian kingdoms of Iberia. Perhaps most famous is the War of the Two Peters, a war of succession that not only involved Castile and Aragon, but the major players of the Hundred Years' War (France and England) and several other kingdoms. But there was also Iberian participation in the Albigensian Crusade in neighboring Provence, various Aragonese conquests in the Mediterranean, and the Aragonese Crusade that resulted directly from the Aragonese conquest of Sicily. Castile actually had several succession crises and civil wars during this period. There were also a number of other wars, which I cannot necessarily call to mind at the moment.

So, basically, there were other priorities during this time frame. Perhaps you or others are thinking "hey, they were relatively easy to defeat during this period, so why didn't it happen quickly?"...but to some extent, it's the other way around: since Muslim Spain was no longer the terrifying threat that it had been in previous periods, conquering that territory often took a back seat to other concerns.

Hope that helps.