Why is it so popular? It's the national sport. It is gaining, not losing popularity.
In the Mediterranean, sacrificing bulls is a practice dating back to pre-historic times. In Greece for example, killing the minotaur is symbolic of a bullfight.
Bullfighting as we know it today, started in the village squares, and became formalised, with the building of the bullring in Ronda in the late 18th century. From that time, it began to follow a particular sequence of events: the entrance of the bull, the picador, the banderilleros, and finally the matador (bullfighter). Many of the picadors' horses were injured in the early days, so these heavy horses now wear protection.
There are about 70 bullrings in Andalucia. Seville is the most important. Don't miss the week of fights which coincide with the Seville Spring Fair. Ronda is the oldest bullring in Spain. It hosts the famous Goyesque Fair in early September. Some bullrings house bullfighting museums. They can also be used for other events such as pop concerts.
These can be purchased from the newsagent in Spain. Popular titles are "6toros6" or "Aplause". They give a good insight into the complexity of the sport.