Why does baking bread in a closed pot make a good crust?
The steam should only be used for the first part of baking. Recipes vary in opinion on how long it's best to have this steam, but the range I've seen is usually 10-30 minutes. The moist baking environment allows the outer layer of dough to remain stretchy so that you maximize "oven spring" in the first part of baking.  Steaming the dough also causes the starches on this outer layer to gelatinize, which is what leads to the crispy and nicely browned crust.  If you continue to steam through the whole baking process, this layer will not have a chance to dry out, which is what's required for it to brown and crisp up.

This article has lots of information on how to achieve a good crust.