Joseph:
No one was questioning why some are motivated to withhold the info about mental illness. The question is whether that is okay according to halacha or yashrus. I doubt that most commenters here believe it is proper to keep such secrets, and I still bet that anyone confronted with such a situation would want to keep it secret.
For those who speak about bipolar disorder or borderline personality disorder, please recognize they are two completely distinct conditions, and rarely co-exist. Bipolar disorder is a treatable illness, though its fluctuations make it difficult to effect a 100% remission. Borderline personality cannot be treated with medication, and its treatment through therapy is partially effective at best. Being married to a borderline (BPD) is one of the greatest nisyonos one can have. Hats off to anyone that can tolerate this life. Approximately 80% of all borderlines are females. There is a wealth of information online about BPD. It is majorly difficult to live with a borderline. It is not fun having the condition either. I have heard people say they wish one could detect BPD in blood tests, so that those with it could be excluded from shidduchim altogether. I know this does not sound nice at all, and perhaps discriminatory. But once you see how these marriages turn out, you will likely agree with that. BTW, there is no blood test for BPD.