Its funny how politics have changed even in my lifetime. When I was younger, while you had Primaries, you could be considered to extreme within your party to be 'electable' and so you generally nominated moderate candidates (from both sides). However, over the past decade and a half, its shifted. If you are at all moderate, then you get beat out in the primary by a candidate that is more 'aligned' to the party principles and less willing to compromise. Now we have 638 members of Congress (House and Senate) who are basically slaves to the party ideologies and are either restricted or simply unwilling to use any common sense to address the real problems of society.
To be fair, the parties have severely punished any candidate that has stepped out of line. If you want a good example of what I'm referring to, just read the 538 article on how the left has shifted its stance on immigration. Just 10 years ago, Bernie called for a wall and voted against immigration reform. Mr. Lefty himself. But I'm not calling this a "D" thing. The Tea Party did the same thing to the Repubs in 2010 and 2012. Anyone who tried to compromise with Obama on anything were railroaded out of office come their next election.
So now we are stuck with two sides who are adamantly opposed to actually working together as its a recipe for being unelectable. And at the end of the day, the only thing the politicians care about is winning the next election.