Fitz' workouts are usually earlier than this, aren't they? It is cutting close to the start of training camp...which starts earlier this year for the Cardinals, and is going to be a six week grind (with trips to Ohio, Kansas City [extended stay] and Tennessee)---so, in that respect, I can see why some players would want to get in their last two weeks some vacation, family and R&R time.
Plus, with QBs they have to be prudent about the number of passes they throw. Bartel isn't likely going to get anywhere near the reps in training camp that Kolb and Skelton will, so in his situation it makes more sense for him to be there.
It might be possible that for all of the reasons above, Fitz himself advised Kolb and Skelton to stay home.