Historical reasons is quite likely. SAS actually does a fair amount of work to make sure that code that worked in a previous version still "works" though some minor behaviors may change. You may get notes or warnings depending on specific code about some of these option.
For example some documentation directly related to your question (from my 9.4.7 online help):
CATEGORY=column | expression
specifies the column or expression for the category values.
Notes
You can use X= as an alternative to CATEGORY=. If you use X=, then be aware that the TIP=, TIPFORMAT=, and TIPLABEL= options recognize X as the category role and not as CATEGORY.
This tells me that if I were using the Tip, Tipformat or Tiblabel options then use of X= instead of Category= may yield some difference in output.
You will see in procedures places where you can specify an option with "alias" names. Sometimes these reflect an earlier version of the options for a procedure that has evolved.
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