Combine this fact with the fact that most enclosed fixtures that were designed for incandescent bulbs are insulated (at least on the mounting side) to keep the

Combine this fact with the fact that most enclosed fixtures that were designed for incandescent bulbs are insulated (at least on the mounting side) to keep the heat from the bulb inside the fixture (fire prevention reasons) and even the low heat level output by an LED bulb can eventually warm the interior above the temperature that the driver electronics can tolerate.