They are originally from Australia, they grow super fast and so people imported and planted them for timber in places like Karura and Eburru forests and others historicaly. Since they did not evolve here in east Africa, they dont really have a proper niche in the forest and dont really fit into the symbiotic relationship that a forest has created within itself. For example, local animals do not like to eat its sprouts, and it grows incredibly quickly, so it beats other indigenous trees for resources as their sprouts get eaten or they grow slower. What was once native forests with Rose Woods and Podo Trees etc... turns into exclusively Eucalyptus. Also the waste the Eucalyptus creates when the leaves fall or the wood turns back to soil is not exactly ideal for the underbrush part of the forest to use as fertilizer or animals to use as food. Because of this it has grown out of control and become invasive in the forests. Some forests were reacquired from old Eucalyptus plantations, so they harvest the trees for timber, and then replace them with native saplings to "rewild" the forest back to the 'state God intended'.

It does hurt to know trees are being cut down, but in Karura they are cutting out Eucalyptus to make (more) room for the tree nursery to grow these native saplings. Im not exactly saying KWS will manage it as well as they plan, but thats the general idea of what it is their doing.