She-Hulk sold poorly enough in its original run under Slott that, while it was critically lauded, it was close to cancellation until the decision was made to relaunch it.
Spider-Girl benefited heavily from several different letter-writing campaigns to save the book, but never found an audience that was quite large enough to justify continuing to put the book out.
I'd also point out that Marvel has done its best in recent memory to attach its new series to some manner of crossover or big event, so at least they start strong. If they don't continue as such, which they don't seem to do unless a given book is visibly attached to one of the big franchises, they pull the plug. A business cannot be faulted for acting to perpetuate itself.