@Heberson-Menezes I think that maybe the reason that they don't is where does it end? Legendaries are the most important monster in the game, so they have to control our access or everyone will have them all. Let's say that given the shear size of the monstergram that they decide to give a new breedable, which element should it be? I have an idea for this, but first let me say that if you add one element, you might as well add them all or players will feel like you are being unfair. Though keep in mind, we do currently have two breedable Darks and two breedable Natures. So in one regard, they have already set a precedent and should continue. Maybe they already have a plan in place to add a new breedable for every 50 or 100 monsters added to the game? Those numbers are just a wild guess, I would hope that they are or will be in fact lower.
Finally if the plan is to release new breedables, they could always have a vote system, where people spend gold or food to vote. This would give long time players decidedly more voting power, but that is fine with me. It would be a loyalty reward of sorts.
Meme-master in his own mind.