Because under the current system the major parties pick candidates based on 'first past the post' primaries. This means that the small percent of the population that votes in primaries has an outsized effect on the outcome. In areas the GOP controls, the usual winning candidate is the most extreme Right winger, because the older folks who vote in every election tend to be more conservative.

Similarly, in Dem areas, the older folks tend to vote more, but they'll pick a bland middle of the roader who they think will be 'safe.'

Any Dem Presidential candidate since 1960 could swap programs with Jimmy Carter, while the GOP has gone from 'peace candidate' Richard Nixon to Donald Trump.

The solution is to get more young people involved in the primaries, but the problem is that young people today have less time and money to devote to politics. In 1960, someone could live well with just one part time job; today it take three incomes to keep two people comfortable.