This answer is so simple that you might just be disappointed: the microwaved food may not have been as hot to begin with.
Many factors, such as the type, quantity and thickness of the food affect how it will heat in a microwave oven. If, for example, the container is not right, or the stirring or rotation aren’t thorough, or if the container isn’t covered to keep the steam in, then the heat may not be distributed uniformly throughout the food.
The outer parts may be scalding hot, but the inner parts may still be relatively cool. Then the foods overall temperature will be lower than you think, and it will cool to room temperature faster.
While heating food on the gas the vessel gets uniform heat from the bottom and it slowly rises heating all the molecules in the food. It is important to stir. The temperature of the food heated this way will be even and will take longer to cool off.
There is another reason. Microwave safe containers are deliberately designed not to get hot. Thus food from a microwave oven is in contact with a container that has remained cooler than the food and that saps away some of its heat.
Chef Sanjeev Kapoor is the most celebrated face of Indian cuisine. He is Chef extraordinaire, runs a successful TV Channel FoodFood, hosted Khana Khazana cookery show on television for more than 17 years, author of 150+ best selling cookbooks, restaurateur and winner of several culinary awards. He is living his dream of making Indian cuisine the number one in the world and empowering women through power of cooking to become self sufficient. His recipe portal www.sanjeevkapoor.com is a complete cookery manual with a compendium of more than 10,000 tried & tested recipes, videos, articles, tips & trivia and a wealth of information on the art and craft of cooking in both English and Hindi.