I expect that there are a lot of reasons for this but two of them stand out to me. One is the time difference. Yeah Joseph Smith lived over a hundred years ago, but he was an American, he lived in places that we recognize on a map. Solomon lived thousands of years ago half way around the world. The culture recorded in the Bible can seen fairly alien compared to our culture today, I think that makes it easier to conveniently not subject past prophets to standards that we submit current prophets to. The other reason that occurs to me is that most people only know about Solomon from the Bible. It isn't the case where the Bible says all these good things about Solomon but the history books say all these bad things. Rather the Bible itself records both his good acts and his bad acts. So they are free to believe all the bad things along with the good and still believe he was a righteous man at one point because the Bible says so. Joseph Smith doesn't have that luxury since he lived in modern times and as such information related to him will come from multiple sources. This makes any mistakes he made seem more real and controversial. As an example consider Nephi in the Book of Mormon. If you believe in the Book of Mormon then does Nephi killing Laban cause you any concern about whether or not he was a true prophet? Or does the fact that the Book of Mormon clearly considered him such, even though he killed Laban, make it a non-issue?