Rashi (from Tanchuma Mishpatim 18): You did not authorize a Shali'ach to pardon - "Ki Lo Yisa l'Fish'achem" (Shemos 23:21). Radak citing his father - Hashem authorized upper beings to do His desire in the land, but only He can pardon.

Malbim: (a) A person (Ploni) who pardons one who sinned against him, it is because the sinner regretted and requested pardon, or Ploni had mercy on him, or he found grace in Ploni's eyes. The reason for pardon is not found with Ploni, rather, with the one that he pardons. Hashem is different. The reason for pardon is in Him. One can create and sustain his creations only if he will always pardon. A Midrash says, Avraham said to Hashem, if You want Din, there is no world. If You want a world, there is no Din. The reason for pardon is with You. (b) Pardon is removing the sin, as if it never were ¹ . Only Hashem can do so. We find Selichah in Tanach only from Hashem.

Rashi: A person will not be confident that someone else will pardon him. Radak citing his father - if angels could pardon, people would say in their hearts, if we sin, we will appease the angels and they will bear our sin.

Ibn Ezra, cited partially in Radak: If You pardon my sin, sinners will hear, and also they will repent. If not, they will not fear You; they will do their desires.

Malbim: If a person wants to be feared, he will not pardon sins, for people fear him only due to his punishment. If he would not punish, there is no reason to fear. People fear You due to Your grandeur. This is revealed via Your pardon. People see that You are so elevated that Your honor cannot be blemished - "Im Chatasa Mah Tif'al Bo" (Iyov 35:6). You are so elevated that no person is a Tzadik in front of You; Selichah causes that You are feared (it reveals how elevated You are).