Because QQueue prepends items whereas QStack appends them. QVector performs worse than QList when it comes to prepending, hence the inheritance choice for QQueue. As for QStack, it would be possible to make it inherit from QList AFAIK but the Trolls probably know what they are doing and there's at list a single reason that motivate their choice : as QVector pre-allocates memory only at its end (contrary to QList) their is no wastage (remember : QStack never prepend on its own, though the coder can do it manually...)

Hope this helps.