Having Smart, Wannamer or Semi out there on the perimeter on offense is more valuable than having Theis space the floor. They are more of a threat (shooting wise all 3, ball-handling and passing wise for Wannamaker and Smart).
Playing Theis and Kanter together makes it easier for Philly to keep Horford and Embiid together. Improves Philly.
Better to keep with small ball. Attacks Philly's weaknesses better. Going big bails them out. Fails to exploit their weaknesses.
Starting Kanter instead of Theis to better deal with Embiid is an interesting idea.