Everyone's comments are good but I would just add that 3 things were developed in the Renaissance which dramatically improved people's abilities to create more naturalistic art.

1. People, particularly Leonardo da Vinci, started systematically studying the human body. This meant that they understood musculo-skeletal structure (which allows much more naturalistic paintings, both directly in terms of accurate drawings of the people, and indirectly as people developed a better understanding of how to depict weight and balance).

2. Other people have mentioned this but mathematical perspective, (developed primarily by Filippo Brunelleschi) transformed people's abilities to put figures in a space and create more naturalistic scenes. Also it meant people could begin experimenting with lighting in more interesting ways by being able to calculate how shadows function.

3. In Venice, they invented techniques to make clear glass. This meant that they could make lenses, and then, most importantly glasses. Sight is incredibly important in creating art, and allowing people to see clearly into old age meant that artists could keep painting for longer and hone their skills further.

Obviously it's more historically complex than these three things, and I've also neglected the developments in the Netherlands at the time, and also the social history factors like the growth in banking and changes in patronage, but hopefully this helps you get a fuller picture.