[lopmotr]

One party has the numbers they need because they need those numbers to be competitive in the current system and have made whatever concessions they needed to to achieve them. If the demographics changed then the parties would adapt so that again they both just barely have what they need. The parties would adapt by campaigning to different groups of voters or changing their policies to attract the other party's voters. It's a waste of resources and concessions to gain more voters than are useful for winning.