Because Oriental Gamefowl were not subject to the same to the same selection pressures as Spanish Game. Spanish Game are matched for weight with bigger birds being discarded. Spanish Game are dubbed and Pea Combed birds are not allowed.
Even with these slight modern differences in selection, which may or may not not have been consistent throughout history, there should be at least some convergence of traits. The basic selective forces are the same or at least very similar. Shouldn't the Spanish have developed greater robustness and strength towards the Oriental type (and I'm not just talking about comb type as this is relatively unimportant in a gamecock), at least in some degree?
Whether the selection pressures were consistent throughout history or not, Spanish Game breeders do discard any large heavy set birds that appear. There is a cultural aspect to this practice (I will not go into the practical reason for match weights on BYP), the Spanish like birds that "compete" in the air. More muscle and strength produces birds that "perform" on the ground. The historical use of Gamefowl is entertainment not just competition and cultural differences produce different rules and preferences that are in effect different selection pressures.
In answer to your question, the birds that you describe may be competitive but they would be of little interest to the Spaniards who value birds that compete in the air. Too much muscle and the birds do not get off the ground. Simply put, the Spanish don't breed those birds because they don't like the way they perform (even if they did win).
You cannot say they are exposed to the same selection pressures when Oriental Gamefowl breeders allow Pea combs, do not dub and do not breed to match weights (giving larger birds an advantage).
Not dubbing doesn't automatically produce a pea-comb, the genetics must be there to start with. My inclination would be that oriental gamefowl breeders don't need to dub as the majority of fowl were pea or walnut combed to start with. If their fowl were single-combed they would.
I agree with the first part of your statement but Pea-comb was available very early in the history of European fowl and I can only assume that there was a cultural reason for not having the trait in gamefowl. For example, peacombed birds might have been seen as defective while cockerals with large single combs were seen as masculine and therefore better fighters.
Not breeding to match weights doesn't necessarily increase size, the small Asil is an example. Obviously cultural prejudices also play a major part.
Speaking simplistically a good big one will beat a good little one. The heavy muscle seen in Oriental Game is a definite advantage when there are no match weights or culture wanting birds to perform in a certain way. I don't know of any Asil that run into 2lb range.