The reason was Harun (as) was a prophet whilst Musa (as) was a messenger.
The distinction is important and is detailed here. In a nutshell prophets (anbiya) brought revelation to nations or peoples that were receptive (e.g., Adam, Ismaeel, Ishaaq, Suleiman or Yahya). Messengers (rusul) were sent to hostile nations that had abandoned revelation (e.g., Nuh, Hud, Saleh, Lut, Ibrahim, Musa, Isa, Yusuf and Muhammad). Prophets are not messengers, but messengers can be prophets, where they gain followers who are receptive to their message.
Harun (as) was a prophet speaking to the Jews and guiding them. Musa (as) was sent to the Egyptian nation, represented by the elites, with whom he interacted. Elites generally leave those who do not question power or authority alone as they pose no danger or threat.
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