The key word being "playing". Perhaps the championship clubs that were in for Shinnie (allegedly) didn't convince him he would be a regular starter. Even when he finishes this new 3 year contract with AFC, he will still only be 26. Plenty time for him to develop further and peak.
Regards moving to the EPL, from the SPFL, via the Championship: I'm struggling to think of a single player who has earned that series of transfers.