Can someone tell me why using Indian Mascot offensive?

It isn't.  It should be seen as a flattering honor, a tribute to the
bravery and fortitude of those whose land we stole.  Some people just
take political correctness to an absurd extreme.

I agree with Nelson, but it could depend on how the mascot is
represented.  Say, if a sports team has someone dressed up as an
Indian dancing around with the cheerleaders, members of "the First
Nations" (as the Canadians say) could take offense, especially if the
costume was incorrect, or the person carried on in a way that that
made fun of his role or simply was silly.