I dunno, maybe because Segwit activates on August 2017, 5 years earlier than that? I rummaged through the mailing list and couldn't find any references of people suggesting August 2022 specifically, and the Taproot BIP 341's Deployment section still has "TODO" in it.

Keep in mind that this was likely decided soon after BIP341 was drafted (some time back in 2019) so my gut feeling says that they wanted to make sure everyone got enough time to upgrade.