Maybe because eucalyptus oil is quite toxic:http://www.ansci.cornell.edu/plants/med ... calyp.html
TOXICITY: Eucalyptus spp. contain high levels of phenolics and terpenoids which can be toxic. Several adverse reactions have been attributed to the use of or contact with Eucalyptus oils, extracts, and fresh and processed plant material.
Overdoses of the oil in humans cause gastro-intestinal burning, abdominal pain, vomiting, and convulsions, depress respiration and the central nervous system, and may lead to comas and death.
Based on rodent studies, the oral LD50 (the orally consumed dose that is lethal to 50% of the animals) for eucalyptus oil is very high -- 4.44g/kg body weight(BW) for rats and 3.32g/kg BW for mice.
Eucalyptus oil is not believed to be teratogenic or contaminate milk. However, some studies have indicated that citronellal and phellandrene, which can be found in some Eucalyptus spp., are weak mutagenics and carcinogenics, respectively.