Only 4 call ups allowed after trade deadline. Couldn't do it if they wanted to... and why would they want to? Do you really want to squeak out an extra win or two to drop us from, say, 5th to 7th like in 2008 so we can spend extra assets moving up to a spot we could have by just staying the course?
Also, calling up Kadri and Colborne weakens the Marlies... their playoff run gives those two plus Ashton, Mueller, Blacker, Holzer, Scrivens, Owuya, amongst others some valuable playoff experience.
If this is a serious question, it's because you're only allowed 4 callups after the trade deadline. The Leafs sent Gardiner and Frattin down and called them up immediately so that used two of them. Then they called up Rosehill and Ashton.
No more callups available unless there's an injury.