The US media landscape is a post-apocalyptic mess. The loss of newspaper subscriptions and classified ad revenue means that the budgets of US media that aren't click bait or internationally known is tiny, and even big internationally known media are struggling.
Journalists jobs are to make revenue for their media company. And looking into politicians backgrounds will usually not generate a pile of revenue. So they don't have the time.
A small local paper did it, and found some smell. But it was one race, and it wasn't a guaranteed story (Republican candidate lies about charity? That is a tiny scandal compared to what goes on every day), so not many other people picked it up.
Journalists fact checking? Where is the profit in that? You just say "claimed" next to what someone says and wash your hands of it. Actually verifying something is true takes time and money and doesn't generate revenue. Maybe a few high profile (newpapers who value their reputation) or inefficient workers (why are you fact checking and not generating clickbiat? please remember to post any spelling corrections in the comments!) do that kind of thing.
Party officials job is to gain power for their party and themselves. As they don't have the ability to change a candidate, looking for and finding out dirty laundry of their own congress candidate has little real upside. They don't care if the candidate is a horrible person, they care if they win the seat. And with tribalism run rampant, a candidate for a party elsewhere being a horrible person doesn't even matter that much to voters.
By the time Santos got nominated, any attempt to replace them would bring attention to the race and reduce the chance of winning the seat. Before Santos got nominated, he was one of many 1000s of people, many of them crackpots, running for nomination. "Party machines" sometimes go after crackpots, but Santos was running for nomination in a seat which had a low chance of success -- burning party machine capital on opposing crackpots there was low return on investment. No serious candidate with power wanted that nomination!
Really, the opposing party nominee did drop the ball. They are about the only people with a real incentive to make Santos lose. The fraud information the local paper uncovered should have been a sign they should spend a bit more money digging.
With enough lead time and the extent of the lies, they might have gotten some free press and swung the election.