The reason why they took this route was not explained in early narratives (at that time the existence of La Canela was still taken as fact, so no explanation seemed necessary).
The reason became clear only on the publication of Pedro Cieza de León's history, which remained in manuscript form until 1871.
Gonzalo Pizarro and his followers left Quito on February 1541, a month before Orellana, who was able to bring 23 men and several horses.
They built a boat, and in February 1542 decided that Orellana, along with 50 men, should continue sailing down the Napo in search of food for the whole party.
The best sources for the expedition are the history of Garcilaso de la Vega, el Inca (Garcilaso grew up in the household of Gonzalo Pizarro and therefore heard his story firsthand) and the narrative of Orellana's chaplain, Gaspar de Carvajal, who took part in the navigation of the Amazon.