Why did Allah send human messengers to people instead of sending angels?

It is more appropriate to send mankind human messengers to talk to their in own language and act as role models for them. Had Allah sent an angel to mankind as a messenger, and he did what they found difficult to copy, they would have argued that the angel could do what they could not.

{Say, “If there had been angels walking on earth peacefully, We would have certainly sent down to them from the heaven an angel as a messenger.”} [174] Surat al-Isrā’: 95.

{And if We had made him an angel, We would have surely made him [appear as] a man, and We would have caused them confusion just as they are confused.} [175] Surat al-An‘ām: 9.