Well, because if we wanted to have a rational discussion about the fact that a vast majority of homicides are committed with handguns, and really why do people need handguns when studies have shown that they are far more likely to cause harm to a family member, or to be used to threaten a family member, or to escalate an argument from minor violence to a homicide, then the "gun rights" people would have some force behind the argument that we're trying to take their guns away. Because they'd rather have access to an easily concealable cannon that can, and frequently is, used irrationally rather than forsake ownership of something that dangerous in order to truly attempt to cut down on gun violence.
The assault rifle just has no viable non-military use that anyone has cogently argued, so it's an easy and valid target to ban. It doesn't matter that it's not the weapon that has been used in a majority of crimes: there is no reason for a civilian to own them, and even the fervent pro-"gun rights" Antonin Scalia conceded that states and the federal government could conceivably ban military-style weapons without running afoul of the 2nd amendment (never mind the fact that such a statement seems to run directly counter to any reasonable interpretation of an amendment that starts with a discussion of military force, but whatever.) Sometimes you have to pick your battles, and this one seems like it could be the most reasonably likely to gain even a modicum of support.