Because they lack control, suggests Nyhan:

In every other aspect of the campaign, the candidates and their messages are filtered through journalists who are reticent to allow them to speak or be quoted at any length without interpretation or analysis. While scrutinizing policy proposals and fact-checking their claims can be valuable exercises, far more coverage displaces the candidates’ messages in favor of ill-informed horse race analysis and theater critic-style analysis of the “optics” of the campaign. Unlike the debates, which are moderated by journalists, the conventions allow the parties and the candidate to speak to voters unfiltered in prime time. That may be threatening to the professional status of journalists, but it’s good for America.