When Akechi Mitsuhide attacked, Nobunaga just happened to not be at a castle and was only travelling with a small entourage. The closest significant Oda forces were at least two days march away. Obviously Nobunaga was not expecting Akechi Mitsuhide to betray him and obviously that's why Mitsuhide chose to attack Honnōji.

The Oda was not one betrayal away from collapse, however. It wouldn't have been surprising if they were, since this was the Sengoku and betrayal was everywhere, but the Oda was betrayed many times before already, and was able to defeat the traitors previously without problem. In fact, you could say that even Akechi Mitsuhide's betrayal was defeated by the Oda, and in a mere eleven days.

The problem for the Oda was that Nobunaga's heir, Nobutada, was also killed in the coup. Nobutada's son, Nobunaga's grandson, was merely two-years-old but was designated heir by primogeniture. Two of Nobunaga's younger sons, Oda Nobutaka and Oda Nobukatsu, vied for power, resulting in the Oda clan not having a united authority. Hideyoshi was successful in convincing Niwa Nagahide, Ikeda Tsuneoki, and many other Oda retainers to side with him. In 1583, Oda Nobutaka tried to oust Hideyoshi with the help of Shibata Katsuie, but they were defeated. In 1584, Oda Nobukatsu tried to enlist Tokugawa Ieyasu to oust Hideyoshi, but while Ieyasu was able to defeat Hideyoshi at Komaki-Nagakute and kill Ikeda Tsuneoki and Mori Nagayoshi, Nobukatsu was betrayed by his own vassals and had to surrender to Hideyoshi. Niwa Nagahide died in the first half of 1585. Meaning by 1585, the main Oda vassals were dead (Shibata Katsuie, Ikeda Tsuneoki, Mori Nagayoshi, Niwa Nagahide) defeated (Sassa Narimasa), or were convinced by Hideyoshi by diplomacy or force to side with him (Maeda Toshiie, Takigawa Kazumasu). Hideyoshi then got the court to declare him Kanpaku (Imperial Regent), meaning he significantly outranked the remaining members of the Oda clan.

So what caused the Oda's power to collapse was not the death of Nobunaga itself, but that the event also resulted in the clan being lead by an infant, giving a vassal the opportunity to create ties and alliances with other vassals, defeat any opposing vassals, and then acquire official authority to cement his power. This process isn't all that rare in the Sengoku, and Tokugawa Ieyasu will do the same thing to the Toyotomi after Hideyoshi's death.