The correct answer to this open question is the following.
Although there are no options attached, we can say the following.
The factor found in the rainforest ecosystem that makes it unlikely that a cactus would be able to live in the rainforest is the excess humidity and lack of sunlight due to the excesses of vegetation in the rainforest ecosystem.
In the rainforest, rain is constant most of the time. That is why it is full of vegetation. Everywhere you look around, you see green and smell humidity. Water is what makes the rainforest the exuberant place it is with plenty of trees and plants. It is a paradise for insects, reptiles, and wildlife in general.
That is why cactus would not be able to live and survive in these conditions. Humidity and the lack of sunlight due to the excess of vegetation are the main reasons.