A reply to /u/MrYeulew

There were several major reasons as to why it took a long time.

First is the simple fact that Castile's rulers often had to either deal with internal turmoil (see the various crises of succession), or they saw an easier picking elsewhere. Examples include the acquisition of Leon, Galicia, etc. over the centuries.

Second is that campaigns of the Reconquista tended to be seasonal, royal control over nobles tended to be limited, and siege weapons were limited. This limited the speed at which conquest could be executed. And this was why the speed of conquest accelerated when Isabella and Ferdinand were able to obtain modern siege guns.

Third is that the final push against Granada, in 1482-1492 was conducted at a very deliberate pace. Isabella strongly wanted to reform the internal rule of Castille, to increase royal control over nobles and the Cortes Generales. Therefore, instead of the traditional method of mobilizing nobles' forces and thus being forced to reward them both in political and territorial powers, Isabella preferred an approach where the war was very strongly under royal control. Thus, she relied mostly on royal resources, which took time to be developed. To a large extent, she succeeded, thus enabling her Habsburg successors to harness the resources of Castile to build their European and worldwide empire.

One of the big reasons why Castile didn't invade earlier is that the Emirate of Granada is located in a relatively hilly area which makes invasions difficult to undertake. As well, there was also always the possibility that Granada would receive reinforcements from Muslim North Africa which would make the invasion much more difficult.

EDIT: I should also add that the reconquista was not as a religious as a conflict as many portray it. Though there were definitely strong religious reasons to expand at some points the region was largely dominated by a struggle for power between the different countries on the peninsula.