Emes L’Yaakov (Bereishis 49:7): ... Look at what I wrote previously in Bereishis (34:5). There I explained that it is certain that that this represents a punishment for Shimon and Levi, nevertheless Yaakov specifically appointed these tribes to these exalted responsibilities of educating the children and supervision of holy food. That is because only Shimon and Levi manifested the kano’as (zealotry) required for these jobs. This was seen by their response to Yaakov concerning the rape of their sister Dinah (Bereishis 34:5): “Is our sister to be a prostitute?” They alone were referred to as the brothers of Dinah because they were willing to sacrifice themselves for her welfare. Thus for these positions to be fulfilled successfully it is necessary to appoint people who have the attribute of kano’as (zealotry) and mesiras nefesh (total devotion) to serve G-d and His Torah. These positions require men who when they see something irregular, will immediately be aroused for the sake of G-d like an inferno and they will serve their position well. Therefore Yaakov specially appointed Shimon and Levi for this work.
However if we look carefully to see whether they in fact did these jobs according to their father’s expectations, we see that only the tribe of Levi actually did the work of education but not the tribe of Shimon. It was only the tribe of Levi that served as a strong barrier against those who arose to nullify the covenant with G-d. In contrast we don’t find that Shimon was involved in this work. The reason for this is clear. Only the tribe of Levi was exempt from working for Pharaoh during all the years of slavery. Thus only the tribe of Levi was able to sit and be immersed in Torah study. Only when kano’as (zealotry) is grounded and defined by Torah parameters - is it successful. That is why when the Leviim saw the terrible chilul HaShem that resulted from the sin of the Golden Calf they immediately expressed their zealotry for G‑d and killed even friends and relatives who were participating in the sin. Similarly when there were Jews in the Wilderness who wanted to return to Egypt, it was the Leviim who were in the vanguard against them. During the entire time that passed from the events concerning Dinah until the Redemption from Egypt they maintained their zealotry - but it was always bounded and guided by their deep knowledge of Torah.
In contrast, while the kano’as of Shimon was also strong it was not guided by Torah and thus did not have proper nature. When did his “kano’as" manifest itself? It was in the events of the worship of Baal Peor. It was Zimri who expressed this “kano’as” against the rule of Torah provided by Moshe. It was used to uproot the attribute of modesty which was then prevelant amongst the Jews. He expressed his zealotry by his open involvement in sexual immorality with the non‑Jewish woman. Therefore it was only the kano’as of Pinchus, from the tribe of Levi ,who was guided by the laws of the Torah who was able to attack Zimri and defeat him and stop the plague that was punishing the Jews. Kano’as which is not guided by halacha does not have the ability to be successful because it ultimately just destroys the kanoi. It is only the gedolim in each generation who have the proper intuition and sensitivity to know and decide when it is necessary to be a kanoi and to protest and when it is best to remain silent. This ability is only acquired by those who have the keys to Torah in their hands.