It's easy to see them as foolish with all of the information we have today. When I think what life was like on the frontier in the 1830's, I gain a whole new respect for them.
They lived on one end of a vast, unexplored continent inhabited by Natives. They wondered how the Natives got there. They had no knowledge of a land bridge from Asia. Their main source of information was the Bible, so many assumed the Natives were from the Lost Tribes of Israel. Joseph Smith seized on this and created a compelling story that confirmed their thinking.
Christianity was their default setting. God, Jesus, angels, and miracles were already ingrained in their thinking.
They lived in a time when many of their children would die of disease before they reached adulthood. Priesthood Blessings and the doctrine of eternal families would have been their only hope. This is hard for us to understand in an age where good doctors, antibiotics, vaccines and health insurance are readily available.
Many of them were farmers. A bad crop or a harsh winter could have been deadly. There was no social safety net, no food banks, no unemployment insurance, no social security. Attaching yourself to a tight knit group that looked out for each other would have had tremendous survival benefits.
It was dreadfully boring back then. People passed the time by telling stories that became grander with each telling. This was considered to be normal. Tall tales were the only type of tales they were used to. You couldn't look back over records and see how the story changed with time. Even the newspapers used flowery, exaggerated language. A book that came from buried Golden Plates written by the ancestors of the Native Americans would have been infinitely more exciting to them than Reddit is to us.
"Visions" were probably a much more common experience back then. The summers were hot, they didn't have air conditioning, and the work was mundane. It would be pretty easy to doze off for a moment while plodding along on a horse on a hot summer day, dream a bit, and awake without ever realizing you had fallen asleep.
Schools didn't teach science back then, and it wasn't part of the average person's experience or thinking. Knowledge was gained through books and the "testimony" of others, not from the scientific method. This is really hard for us to understand in a time when we check sources and require double-blind studies. Much of our scientific knowledge did not exist back then.
For instance: The Book of Mormon was published in 1830. Gregor Mendel began his work on Mendelian Inheritance in 1856 The Origin of Species was published in 1859 It is hard for me to imagine how different my worldview would be without these basic foundations of scientific thought.
Joseph Smith hadn't yet been caught in any of his huge and obvious lies. I'm sure he was a very convincing liar.
The polygamy stuff was crazy, but most of our ancestors wouldn't have known anything about it until they were isolated in Utah and trapped by Brigham Young's theocracy.
None of the evidence against the church was available in mid nineteenth century America!
So what makes my pioneer ancestors different from all the other people at the time who didn't believe? I suspect they had hope, an infectious yearning to be part of something bigger than themselves, a desperate need to feel that they were making the world a better place. I'm guilty of this myself, and it has helped propel me out of the current church, in search of something better. This same desire fuels people like John Dehlin, Kate Kelly, and the people on this sub who organize meetups. It might be foolish, but it also might be worth it.