Because you overmixed the batter. When making any type of muffins, it's very important to use two different bowls. That way when you combine your dry into wet, you are simply just folding the two sets of ingredients together. They have already been mixed separately. Another reason they could be dry is because you overbaked them. Start watching your time at 18 minutes. If they aren't done, then put them back in for a few minutes at a time.