When the lightbulb was invented, it wasn't very long before nearly every home and building in the world was using them. Why do people use them? Simple.
Because they work.
Why are there 10 million different religions all claiming to have the truth?
Because they don't work.
If there really was one true religion that "god" favored above all others, then it, too, would "work". This one true religion would flood the world, just like the lightbulb did, and everyone would use it.
But no religion in the world has ever done that. Religions grow slowly. If there was a one, true religion, then it would fill the world. People would line up by the millions to join. Let's say that the LDS church really was true:
- Everyone who paid tithing would always and immediately have a huge increase.
- Everyone who received a priesthood blessing would be healed.
- Angel "sightings" would be commonplace.
- True miracles would be common.
- Prophets would correctly predict the future.
Mormonism claims that it has all these things, and yet it actually has none of them.