Well, what is so bad about outlawing libel against the government?
A government is not a person. How can it be libeled?
The original point of libel laws was to defend private individuals from having their character defamed in print, on the presumption that few private individuals had the financial means to defend their reputation against a publishing concern.
The government has the resources to defend its reputation in the court of public opinion.
Moreover, there is a conflict of interest when the government, as the offended party, is also the prosecutor and judge.
Finally, there is no bright line between expressing an opinion and making a factual claim. Language is fluid. It is better to err on the side of presuming opinion, in order to give freer rein to free speech.