Why can't a utensil that became trief at, say 150 degrees be kashered with water that is 151 degrees? Why must the water be boiling?
R Ezra Friedman from OU Kashrut brings an answer to your question (here)
It would seem logical based on the principle of kebol’o kach polto that not only would the method of kashering be based on the form of absorption but the temperature as well. For example, if a utensil absorbed non-kosher soup at low heat, one might assume that the hot water used in the kashering process to remove the absorbed flavor would only need to be slightly hotter than the temperature that the soup was cooked at. However, it is clear from early authorities (see Rabeinu Yona Hagalat Keilim 40) and codified in the Shulchan Aruch and Rema (OH 552:1) that this is not the case. In order to remove absorbed flavor from the utensil, the water must be boiling hot, even if the original absorption occurred at a lower temperature.
This concept is explained by Rav Aharon Pfeuffer in his book Kitzur Shulchan Aruch Basar Bechalav (volume 2 appendix:1). When our Sages codified the rule of kebol’o kach polto, it related to the form of absorption such as water or fire and not the temperature needed to remove the flavor. In order to remove absorbed flavor, a higher temperature is needed than the temperature in which the food was cooked. The temperature of the water must be boiling in order to remove all of the flavor. It should be noted, however, that Rav Moshe Feinstein (Igrot Moshe YD 4:36) and other poskim ruled that in situations of severe need, hag’alah may be done using water only slightly hotter than the temperature at which the food was absorbed. The OU does not rely on this leniency.