Both parties have weak fields (from a non-ideological perspective, just in electoral ability) for very different reasons.
The Republicans have spent decades developing a culture where nobody can remain relevant inside the tent while calling out the bullshit that people who are passionate but don’t think very hard can be led to believe. The Democrats have just as big a proportion of passionate people who don’t think very hard. But if you pander to them more than just a little bit, there’s no state in the country where a Democrat can expect to be nominated for governor or senator.
The Republicans, though, insist on candidates who believe, or give a very convincing show of believing nine impossible things before every spaghetti breakfast, Rotary Lunch, and rubber-chicken dinner. And at those functions, no one will question the nonsense. So the candidates are all either dumb enough to actually believe the bullshit or are habitual liars who are unpracticed at selling the lies before skeptical audiences.