You're telling me what the problem is. But you're not explaining why it's a broken system.
I think a big problem right now is the primary system. This topic is about Graham Platner, but let's not ignore the fact that Ken Paxton got the Republican nomination over in Texas despite having plenty of past scandals himself. And speaking of Texas, Paxton is a weak enough candidate that Democrats might actually have a shot at picking up Texas... except their nomination, James Talarico, has a lot of his own baggage (though not so much "scandals" as it is saying some kind of dumb-sounding stuff and a history of positions that are very unlikely to be popular in Texas). When a Democrat managed to win a Senate seat in Alabama, while part of the reason was the Republican candidate (Roy Moore) being extremely weak, another important factor was that the Democratic candidate (Doug Jones) was reasonably moderate and thus better able to appeal to conservatives repulsed by Moore. Not having a candidate you can at least reasonably try to portray as moderate as the opponent may cost the Democrats a winnable seat here. Maybe Talarico wins anyway, but he makes it harder.
Anyone whose goal was to try to win an election with someone reasonably faithful to the party's platform would have looked at these guys and said "yeah, no" and looked for someone else (granted, if they had gone with the other main candidate for the Democratic nominee over Talarico, they still would've had an uphill climb given I think she was more liberal, but there presumably would ahve been more people to look at if it wasn't a primary). But that's not how primaries work.
While the following was specifically written
presidential primaries, it really applies rather well to smaller ones also:
...
In order to succeed in caucuses, candidates have to make a strong, deep case why they’re the right candidate to carry the party banner, and they need to command a loyal, competent organization.
Primaries are a lot simpler. To win a primary you need only two things: high name recognition and massive amounts of money. You aren’t trying to persuade deep, thoughtful, experienced partisans that your positions are right on the merits and that they can win over a general electorate. In a primary, you’re trying to excite/infuriate low-knowledge, low-interest voters so much that they’re willing to go stand in a booth for 12 seconds to fill out a ballot for you. Just like in the general election, primaries are more often decided by gaffes and memes than by actual substance. In a primary, the soundbite is king. Also: money. Gobs of money. You need to plaster your face on every gas pump ad screen from Dixville Notch to Seabrook Beach so those couch potatoes know who to vote when they roll themselves to the polling station.
And that's exactly the problem. A primary isn't about persuading people who are actually strongly committed to winning elections for the party, it's to get out the more extreme people to come out and vote for you to who the desire to actually
win the general election is secondary. And a lot of people just sit out on primaries! Guys like Platner, Paxton, and Talarico would have immense difficulties winning in a convention or caucus system where you have to actually persuade people who have a vested interest in the party and winning to vote for you, but primaries lets weak candidates go forward.
Regrettably, I don't know what the right fix is. Once you give voting rights, it's hard to take them away and switch back to the older and more restrictive ways of choosing candidates. Maybe more states should just set up something like Alaska's (relatively new) system of letting as many people run at once regardless of their political affiliation in the primary, then have the top 4 winners go to the general election and have it decided by instant runoff voting. While I don't think that's perfect either, it seems to at least address this issue. If we're going to have the nominees for parties decided by popular election, at least put the entire process by popular election so that the people who only vote in the general election--which is most voters--can have some sort of say in which Republican or which Democrat wins.