Why is Morality Subjective?
First, That question supposed to dwell on Philosophy?
Second, Religion and Beliefs are prerequisite for Morality
Third, It is subjective because, what is considered morality depends on the collective beliefs of a Religion or a Cult. Behaviors and Actions that are accepted or practiced by a certain group of believers, Are identified as moral for that certain group.
However, What is considered Moral for a certain group of people, might not be accepted or considered as Moral for another group of believers.
Example: A specific religious group considers Polygamy as Moral and acceptable, as long as you are able to provide, however a different religious group, who practice monogamy, considers Polygamy as immoral and outrageous.
In Simple term, Morality is Subjective, because is varies from one group of individuals to another.