Could you clarify your question a little further? I'm assuming you are referring to the initial arrival of cortez and his allies and not to conquest of the city (since refusal of entry wouldn't matter too much during a seige).

While we cannot exactly know what was the logic behind Motecuhzoma’s decision (because we do not have first hand accounts written by him) we can speculate what he may have based his decision on. If you put that action into the context of Tenochtitlan, one realizes that the Aztecs had the upper hand because of the sheer number of inhabitants. While the Tlaxcallan force largely increased Cortez’s forces, they were nowhere near the almost 140,000-200,000 people living at Tenochtitlan. From a military perspective then, it was a poor decision to enter a foreign city without clear knowledge of what was ahead, especially with the natural defenses that Tenochtitlan had. So, I think that Motecuhzoma was strategically trying to monitor the deeds of the Spaniards to assess their deeds, and that of their Tlaxcallans allies. That is why, along with the diplomatic principles for how to treat the representative of foreign leaders, he allowed them to enter the city and housed them at the Palace of Axayacatl in order to supervise them.