“Why is the vision in one eye worse than the other?”

There are so many answers to this question and it would be inappropriate to attempt to answer over the internet. Please see an eye doctor.

Everyone has a dominant eye that may be slightly better than the other. If there is a significant vision disparity there may be a medical problem with the weaker eye. You would need to see an eye care professional to find out. If this is of recent onset you should get it checked out as soon as you can.