The illegal jirga ("i" pronounced as "i" in "bit")system is popular because it administers speedy justice, although the method of determining guilt is sometimes very strange. Sometimes the accused is made to walk or run over burning cinders of coal to prove his innocence. The verdicts are also very quirky. If a man is found guilty of having illicit sex with his neighbour's wife or daughter, it's his wife or daughter or sister who has to pay the penalty (sometimes she's stripped naked and paraded through the streets of the village, as happened last week). In murder cases, the victim's father gets to marry two minor daughters of the killer, even if the poor girls are less than nine years old.
So the courts very properly declared the jirga system as illegal. But despite this, the jirga system remains popular, mainly because it enjoys the support of the feudal lords of Pakistan (the ones who sit in parliament and don't pay taxes).
But another reason for the popularity of the jirga system is the inordinate delay by judges to decide cases. I know a man who was able to retrieve his plot of land from land grabbers after spending 30 years and a couple of million rupees out of his savings. I'm personally dealing with a simple inheritance case which was filed nine years back and hasn't progressed much. Although the plaintiff (my cousin) was cross-examined two years back, the defendant has yet to take the stand, mainly because the defendant's lawyer is well-connected and manages to get a new date of hearing every time. And as if that weren't enough, all the seventeen High Court judges go into hibernation during June and July every year for summer vacations. No wonder the common man has lost faith in the judiciary.