Most fruit pies call for two crusts, whereas most custardy pies do not. However, my apple pies with one crust or two or substantially the same.
Is the number of crusts simply traditional?
Yes, it is mainly tradition, appearance, and how you like the ratio of crust to fruit in your pie. You can make any pie open faced, with a full top, or any type of lattice top. The other thing to consider is that a top crust provides some heat insulation to the fruit, so if you don't want the fruit to caramelize as much, it will help with that.
I think it's traditional to some extent, but double crust pies have several advantages for fruit pies. For one, it keeps the fruit covered, so the fruit can simmer, much like cooking in a pot. Second, it keeps the bubbling fruit from seeping over the edges of the pie plate. Third, a top crust adds another surface to keep the pie together when it is being sliced. More surface tension means more structurally sound pie slices.