it's a power rivalry, and not to make too much of it there are personalities involved too, as well as tactics
here is something I saw that is helpful
http://thehill.com/policy/defense/218387-five-ways-isis-is-different-than-al-qaeda
ISIS is fighting more like a conventional army than al Qaeda ever did.
It has seized territory and declared a caliphate in northern Syria and Iraq. ISIS is also employing more conventional military tactics, using assault rifles and grenades.
Al Qaeda has not sought to hold on to territory as much as it has been focused on carrying out spectacular attacks that would seize attention from international media. Since the attacks on the Twin Towers and Pentagon it has continued to carry out high-profile, complex attacks, such as last year’s assault on the Westgate Mall in Kenya.
Al Qaeda also wants to establish an Islamic Caliphate, but it has taken a long-term approach, and has discouraged affiliates from going out and doing so.
This is also one of the major reasons why al Qaeda disowned ISIS — it wanted ISIS to wait on establishing a caliphate.