The Will may have been contested and it took that long for it to become finalized. There may have been more than one Last Will and in the end a judge has to decide which one was valid.
There are legal wranglings by family members and their attorneys (solicitors) to make it a long drawn-out process (and in the meantime, family members die so it all goes to one person).
It may have taken that long to make sure all assets were found and liquidated into the Estate if the person didn't have all of his affairs in order at the time of death. (The surviving members of the family have to hunt down and locate those assets).
There might have been economic hurdles to overcome as there are many fees to pay in order to enter a Last Will into probate; court costs, et al. Would the Estate have enough money to cover those costs or did fees have to be paid upfront by the surviving family members?
Those are just a few of the possible reasons ...