Care if I hazard a guess?
1) Carlos II's will. He preferred Anjou to the Habsburg candidate. Partially because he was still in love with his first wife and partially because the Austrian ambassador was a cheap bastard (at least in comparison to his French counterpart, Harcourt, who came under Louis XIV's instructions to make the road for the French as smooth as possible in Spain)
2) Seniority. Karl's claim is pretty low down the totem pole (compared to Anjou or the prince of Bavaria). The French claim came through the elder sisters, while Bavaria's were through the younger sisters. Karl's was also through the younger sister, but only through his paternal grandma.
Unless you're talking about in England itself.