Simmonds hadn't played a game since the beginning of February when he caused some sparks during a matchup against the Boston Bruins. Following that game, he was placed on and cleared waivers, with the Leafs assigning him to the Toronto Marlies.
It was never announced that Simmonds was an official call-up from the Marlies, so for him not to be part of the lineup can't be that surprising despite him being a full participant in practice. With all of the new additions to the team, and the Leafs are still trying to learn to mesh together, I think Simmonds can easily be brought in for a veteran aspect. Having something like that during practice can be crucial when learning to play with one another. Either way, it's clear that Simmonds brings much more to this team than just his presence on the ice. He still plays an important role for them off the ice, as does the injured Jake Muzzin.
Instead, the Leafs will be rolling with 11 forwards and 7 defensemen once again. It's not the most popular setup, but it does ensure that the king's ransom of defenders that the Leafs have rostered can all stay fresh and loose.