Because of the connotations that people attach to those words greatly differ. Like to be humane is generally compared to being civilised, decent , being in a functional cognitive state whilst the connotations of the phrase animalistic have the properties of being feral, being volatile ,irrationality ,having a cognitive impairment or just generally behaving like animal , which is often interperatated as an abnormal trait for humans .
Why does the word 'humane' have a positive definition while the word 'animalistic' has a negative definition?
Because as humans we believe that being human means controlling your instincts and being animalistic means not controlling them.