What is the cause of the boy's late arrival at Araby?

The boy's late arrival at the bazaar called "Araby" is his uncle's fault. The boy can't leave for the bazaar until he gets some money. The adults in the story cannot understand the importance of the bazaar because they are unaware of the boy's love for Mangan's sister. The boy doesn't want to go to Araby for his own sake, but rather because he promised to buy Mangan's sister something while there. He is not only suffering from the pangs of young love, but he is also being tortured because his uncle is unusually late to arrive home. At one point his aunt says,

I'm afraid you may put off your bazaar for this night of Our Lord.

The narrator specifies that these were "the short days of winter," which would make it seem more and more impracticable to leave for the bazaar as time went on. 

At nine o'clock I heard my uncle's latchkey in the hall door. I heard him talking to himself and heard the hallstand rocking when it had received the weight of his overcoat. I could interpret these signs. When he was midway through his dinner I asked him to give me the money to go to the bazaar. He had forgotten.

When the narrator says he could interpret these signs, he obviously means that when he hears his uncle talking to himself and hears the hallstand rocking under the weight of the overcoat, he knows his uncle had stayed late at some pub and gotten drunk. His uncle forgot about his promise to give the boy some money to spend at the bazaar. No doubt the boy would ordinarily have realized it was too late and too cold to be leaving at nine o'clock, especially since it might take him another hour to get there. His promise to Mangan's sister makes him feel he has an important quest and must endure all obstacles to fulfill it. The boy prudently endures another delay, waiting until his uncle is midway through his dinner before asking for the money. He hopes the food will make his uncle more sober and therefore easier to deal with. Evidently, the boy has had plenty of experience dealing with his uncle when he has been drinking. Finally, he receives a coin from his inebriated uncle, who insists on reciting The Arab's Farewell to his Steed.

I held a florin tightly in my hand as I strode down Buckingham Street towards the station. 

A florin was a British silver coin worth two shillings. The train takes an intolerable amount of time to start and then seems to creep among "ruinous houses" to its destination. The boy arrives at about the time the bazaar is ready to close. It is two minutes to ten. 

Nearly all the stalls were closed and the greater part of the hall was in darkness.

The contrast between the reality of the place that calls itself by the exotic name of "Araby" and the boy's expectations is the main point of the story. The boy doesn't have enough money to buy anything, even if all the stalls had been open. 

I allowed the two pennies to fall against the sixpence in my pocket.

He pays a shilling to enter this closing bazaar, and most of the remainder of the florin went for train fare. He had only eight cents left to spend on a present for Mangan's sister. One of the few concessions that was still open seemed to be offering nothing but "great jars that stood like eastern guards at either side of the dark entrance to the stall."