OK, so if the Democrats didn't lose because of their candidates, why did they lose in 2010, 2014, and 2016?
Things like elections involve complex issues. Winners win and losers lose for a myriad of different reasons, some of them intertwined with other reasons. But because, as has oft been said, "all politics is local", meaning each district has its own local issues, one would have to research every contest in question and examine all the facts and factors involved.
You are the one making the specious, overly-simplistic claim that the only reason Democrats lost majorities during those election years is because they weren't far enough over to the progressive, looney left. So it's up to you to do all that research and post the conclusions and the links to back them up.
Until you do that and prove your point, I will continue to maintain that anyone who thinks that the Dems lost majorities just because their candidates weren't liberal enough, is either a naive simpleton or just another disingenuous agenda pusher.
Let me add though, that in some cases and in some races, what you claim may very well be true. But I seriously doubt that across the board and in dozens of Congressional districts across the nation, it was simply a matter of what you claim.
AAMOF, I could just as easily claim that some of the seats lost by Dems in those election years were lost because the Dem candidates who ran were TOO liberal.