Stage races in cycling are interesting because they typically have multiple competitions inside one race. You have the general classification (usually seen as the most important one), and then there are numerous other classifications such as points classification, intermediate sprints classification, mountains classification, young riders classification, and the combination classification.
The combination classification is the one that confuses me, especially in the Vuelta a España, where it has consistently been one of the prizes for many years. This classification is almost always won by the same rider who wins the general classification. Because the combination classification is (literally) made up of a combination of the other classifications, there is no way for a rider to go for this prize; in order to improve your standings in the combination classification, a rider needs to improve his standings in all other classifications, which is something nobody is interested in doing. People who are possible contenders for this prize are usually also contenders for the general classification, which typically means that they only care about the general classification.
Interesting statistic: After 2003, only one rider has won the combination classification without also winning the general classification. This was Alejandro Valverde in 2012.
So, my question: why does the Vuelta keep employing the combination classification, and why don't they switch to a different prize that riders would actively try to achieve? Are there any historical reasons behind this, or sponsor contracts or so?