At the time Maus was published (the mid to late 80s, I think), very few people in America had conceived of sequential art as a valid form of self-expression to communicate anything other than 'BAM!' and 'POW!'. The very thought of describing the Holocaust in anything but plaintext or fine art was also controversial.

Ergo, 'groundbreaking'.

Personally, I found Maus to be fantastic (there are things that one doesn't always pick up on a first read), and I own a copy of the second half.