I decided to take a look at this book's sales from July-November. It turns out the Marvel ongoings that sold worse than Young X-Men (not counting separate imprints like MAX or Icon, or books that have already been cancelled) were Captain Britain, Ms. Marvel, Nova, and Punisher War Journal.
Also, despite sales usually sticking around 40,000, sales dropped big time in November to 30,000. Of course, every comic saw a huge dip in sales around October/November.
I guess they really did only cancel this because of the upcoming New Mutants series following X-Infernus. But if DC can pull off both Titans and Teen Titans, why not Marvel with New Mutants and Young X-Men?