Or was he just too busy with his lead role on Cheers? Before the Frasier spin-off existed, five Cheers characters crossed over to Wings: Frasier, Lilith, Norm, Cliff, and Rebecca.
or he could have been tired, or it could have been scheduling or star power
I wouldn't assume he didn't want to go because he disliked the show, but if he was busy working lot of hours he might not have wanted to work the extra hours. I feel like I read somewhere that the Cheers characters came over to help promote Wings as it was getting off the ground. Those who played more minor roles would have more time to go over and do their guest starring.
Even if Ted had the time, scheduling is also an issue that could prevent him being able to do it. If he always had early call times, he might never be able to do a full guest starring stint if the scene required early call times, too.
Or, they might not have never wanted him there - Ted does take up all the air in the room. He's got so much leading man charisma. Steven, the brilliant character actor that he is, might have paled in the Sandpiper aircraft with him in it?