Ford's testimony cannot be trusted, because her memory is not perfect.
Kavanaugh's testimony must be trusted, because he claims his memory is perfect.
Too bad we k from numerous witnesses that Kavanaugh used to get massively drunk all the time.
So why is her testimony less reliable than his?
While I am sure that there are plenty of right wingers who would give some sort of answer to your final question, it really isn't the right question.
It's not the case that his testimony is "more reliable" than hers. The issue is that there is insufficient data available to judge whether or not it happened. Her story, that she was assaulted by two young men at a drinking party, is not at all unusual, so there's no inherent reason to reject it as implausible. Her memory of events is spotty, but that isn't unusual after 35 years, so there's no reason to dismiss the story out of hand on that account.
Then we get into some more problematic aspects. She remembers two people who were there, other than the perpetrators, but those two people can't remember anything like that party, which gets problematic. We don't know everything they told the FBI, but we know that no one who saw their full testimony said their testimony confirmed Ford's account, while several people said it upheld Kavanaugh's. The same is true of the testimony of "Timmy" and "Squi". That doesn't prove anything, but it sure does seem like a lot of people's memories have some holes in them.
Finally, it still seems odd to me that she remembers where she was before the party, and she remembers where she went after the party, but she doesn't remember anything about how she got to and from the party. Again, that doesn't prove anything, but it's just odd.
Kavanaugh, on the other hand, is consistent, but it's easy to be consistent when you simply deny everything. He could be lying.
So, we have a case where we simply cannot decide whose account is closer to the truth based on the evidence we have. It's not that her story is less credible than his. It's just that neither one offers proof, one way or another. In such a case, I believe we must find in favor of the accused.
Some people have objected that that standard only applies in court, but I believe that is exactly backwards. It is applied in court because that is the standard by which all accusations should be judged. The accuser in any situation ought to be required to provide evidence in order to be believed. Even on these forums we are constantly seeing posters who demand that the person who puts forward a claim should put forward evidence supporting the claim. We are so familiar with the idea that it is frequently shortened to a single word, "Evidence?" An accusation is a claim, and should require evidence to be believed. In this case, the evidence is incredibly thin, and that is if you accept a generous interpretation of the reasons for missing evidence and memories.