Clearly, it was sheer good fortune that resulted in usurpers of caliphate acquiring positions of power and authority in Islam, despite an unimpressive, even damaging track record.
To understand this better, we must take others who were just like the usurpers, but could not acquire caliphate due to bad luck.
Ali al-Wardi, the famous Iraqi historian (exp 1995), has explained this perfectly:
It was nothing but bad luck that caused Abu Jahl to be killed during the Battle of Badr while having been in the line of the polytheists.
Had good fortune helped him, in the same way as it had helped others like him, and saved him from being killed during that battle to stay alive up to the day of the conquest of Mecca and to embrace Islam, he would certainly have been one of the grand Sahabah or the first-class Muslim leaders who claimed having raised the standard of Islam.
Thus, luck was the only factor. Nothing but luck played in the destinies of men so hugely. The examples of such good fortunes are being openly experienced by us every day. We have very often seen how men belonging to the same class of Abu Jahl are taken to the highest ranks by their lucks and are surrounded by reporters and transmitters of hadith who encompass them with haloes of greatness.
Dr Ali al-Wardi in Wu’aaz al-Salatin p 118
The usurpers of caliphate were no better than Abu Jahl. It was only by fleeing from battles that they survived long enough to oppress the true successor of Prophet (s.a.w.a.) by supplanting him from the position decreed by Allah and His Prophet (s.a.w.a.) in Ghadeer.