Money is obviously a big factor, but I would say that more so is the decline of Spanish youth systems.
They are simply not providing the same quality of players they used to be, in comparison to other big names. That used to compensate for financial shortcomings before but not today.
Spain for example lacked a modern CB that is athletic and bully while still knows how to play with the ball, a guy like Eric Garica was almost a starter at some point ffs.
They lack attackers, almost no decent CF there or even a top notch winger has been there for past decade or so, gone are the days were the likes of Torres and Villa were competing and they had Pedro and Silva there too. Even their youth teams lacks exciting attackers and CB for example, only one is Yamal and no surprise they want to tie him to NT at 16 as he is the only one in horizon who might turn into WC attacker from players under the age of 35