Wasn't it pretty well-known that Gustav wasn't playing at 100% early in the year last season?
Abbott is having a great season, but Nyquist was the type of generational talent that isn't common at Maine anymore. Abbott has steadily improved and stepped up to play the bigger minutes (although some of those blue-line turnovers make me go:

) but his defensive game isn't nearly at the same level Nyquist's was. Gustav had the vision to interrupt the opposition, while Abbott isn't quite there.
Abbott does do a better job of getting the puck on (and sometimes in) the net than Nyquist did, but his game elsewhere is somewhat flawed.