First, because it is more than a debate about taste, since there is plenty of politics in all. On the other hand, because there is cavas and champagnes for everyone, so it is difficult to generalize.
Let's say, I am not particularly inclined to cavas or champagnes, the same way I am not inclined to white or red wines. Depends on the occasion, the desire, the money...
Both cava and champagne are elaborated equal, following the “champenoise” or traditional method, although there is some cellar on both sides which uses any of the other existing methods.
In principle, the cava seems the best choice for day to day (unless your everyday life includes eating oysters and caviar). There are plenty of cavas from 3 to 6 Euro with more than enough quality to accompany a meal which combines with a sparkling wine. And I do not talk about the cavas of the great wineries we all have in our heads, but something like Xamfrà, a pretty decent brut.
In the next price range is where cavas really make a big difference. Between 6 and 15 Euro there are cavas for everyone, and with a fairly high level. This does mean that large wineries offer great products such as Anna de Codorníu or Sumarroca Cuvée Gran Reserva, and also smaller wineries highlight with cavas such as the exceptional Fuchs de Vidal Únic.
The dilemma arises from 20 Euro, which is really where the champagnes enter the competition. There are still cavas which resist the comparison, such as the Recaredo Turó d'en Mota or the Kripta, but the amount and quality of French champagnes really obscure and overwhelm any other sparkling wine. Even in Spain is comparable, but in the international market, more open, it is hard for cavas of these price ranges to compete for more than to give color to a wine exhibition.
However, that does not mean that high-end cavas have little or no sense: it is a matter of taste. The Spanish cavas are like a Ribera, a Bierzo, a Toro or a Priorat wine, more powerful on the palate, stronger flavor and with more presence of aromas. And we do not talk at all about unsuccessful denominations of origin. In contrast, French champagnes are like their Bordeaux, very subtle wines, with nuances of the terroir, with very slight details that add elegance to them.
This gives a bonus to the cava. In the eyes of many Spaniards like me, accustomed to mom's cooking, which filled the house with aromas and flavors, the subtlety of Champagne sometimes escapes us, and we require an adjustment period to really enjoy the sparkling French. And we not always want to adapt our noses, but simply enjoy a glass of fine cava.
Those who have worked their nose, the experts, those who are accustomed, those persons who I deeply envy because they have known and tested well and extensively, those end up preferring the champagne. Well, not everything in life is Ferrari, in fact, Red Bull is leading...