Just generally ignorance. You'll see very few people familiar with both sports who engage in that sort of nonsense. However many people who are not familiar with both make the natural, human presumption that the "other" version is somehow inferior and feel the need to comment on that "fact". That and the odd troll who just basically wants to see how many people he (or she) can tick off.
Some American Football players and fans jealously guard their sports perceived status as the "toughest". This is why you often hear them badmouthing Hockey as well as Rugby. The truth is that there are different ways of measuring toughness, and those 3 sports, in some order or another, are the toughest team sports.
I am a fan of Football (American) first and foremost but I have a great deal of respect for Rugby players as well. Neither of the two sports is played by "pansies" they are both very physical, tough, full-contact games that deserve the respect of anyone.