For better or worse, the argument is that an assault on an elected official is also an assault on the people who elected him and the office he holds.
Your point is reflected in the fact there are separate statutes relating to attacks on public officials. Not only for the reason you stated but also because they’re more likely to be attacked than your average citizen. You know, like the Colorado House minority leader who got roughed up by a mob in Denver the other day while the police were on a stand down order