https://wccftech.com/larger-apple-silicon-imac-not-releasing-2021/

Just like how Apple announced the M1 iMac later, the bigger version is expected to land in 2022, not 2021, according to a tipster. While there could be several reasons behind this prediction, one of them might be that the company wants to drive up M1X MacBook Pro sales

While it is disappointing to hear about Apple’s rumored plans from @dylandkt, launching both the iMac and MacBook Pro could take away the spotlight from both products, meaning both categories will not receive the same amount of attention. This could have been one reason why Apple chose to launch the M1 iMac later, and as you know, it sports the same internal hardware as the machines that were launched prior.