It's likely that Google equips the Pixel 6a with a regular 60 Hz display in a bid to keep the Pixel 6 more "premium".
Camera performance has been the Pixel series' selling point, and keeping an objectively premium sensor on the cheaper phone sets it up for success.
If the Pixel 6a retains the Pixel 5a's US$449 price tag, it may well be a better option than the regular Pixel 6, although that phone looks set to be offered with significant discounts these days.