Why did the Hebrews have so much trouble uniting into a powerful political force?

I assume you are asking about the period before the Kingdom of Israel, in other words the period from the successful invasion of Canaan to the rise of the first king, Saul.  This is termed the Period of the Judges, ranging from about 1216 to 1035 BC.  One reason that the people did not unite into a single political entity is that they were organized into 12 tribes, each of which was quite large and divided into clans. The tribe of Judah, for instance, when they left Egypt about 1230 BC, counting just the men over the age of 20 but not including those unable to bear arms numbered 45,650. Many were still nomadic and semi-nomadic during the Period of the Judges, wandering with flocks or herds of animals year-round, or part of the year while farming also. Other settled in communities where agriculture and other trades were practiced, with herding going on in a surrounding area easily traversed by the animals daily.

The structure of Hebrew life was semi-anarchic, and not easily conducive to the sort of organization in which the individual is subordinated to a political ruler. The responsibilities of the individual were oriented toward family, clan and tribe, then to the Israelite people as a whole.  During the Period of Judges there was no central ruler of the nation, but the Bible says "each man did what was right in his own heart."  When there were problems which could not be dealt with in this manner, or by the various laws handed down from the time of Moses, the people had recourse to those individuals considered "Judges."  These were a succession of persons, both male and female, who were considered especially blessed with wisdom.  These judges were not chosen by the Israelites, but became known as leaders through their actions.  In this way, the Hebrew people believed God chose these judges and intended them to be listened to, as the earlier people had listened to Moses and then Joshua.  Examples include Deborah, Samson and Samuel.

The entire structure of Israelite life mitigated against a central human authority.  It was not a theocracy, but a sort of moralistic anarchy. Because of the rise of the nations around Israel, the people sent delegates to the last Judge, Samuel, and asked that he ask God to appoint them a king.  If the history recorded in the Bible is correct, they did this not because of fear of the surrounding kingdoms or any need for a more centralized political power, but because they were jealous of the prestige and power of those kingdoms, and the luxury in which their ruling classes lived.

Samuel pointed out that the people of Israel lived free of the taxes and persecutions of kings, and drew their attention to the slave-labor, taxation and other powers over the people that a king would demand.  In short, he told them it was a bad idea, but they insisted.  This led to the first failed king, Saul, and eventually to the powerful King David and his son Solomon.  The people of Israel did indeed suffer from the taxation, enforced labor and various other depredations common to kings throughout history, which eventually led to the splitting of the kingdom about 928 BC.

In other words, it was not so much that they "had trouble" uniting as that they had no cultural reason to unite.  The urge to create one political entity as a unified kingdom came about because of desire for power and prestige, not an actual need.  The trouble afterwards was that their first choice as king was too weak and self-centered to create a strong kingdom.