Because it is a platform governed by a 3rd party entity in a foreign country. That platform can ban and censor citizen, based on foreign cultural values and arbitrary rules, limiting citizen access to their own goverments information.
The platform governments choose to use for public information and debate should always provide open and public access to that information.
A government should not require its citizen to create a Twitter account, and thereby requiring them to provide their personal information to a foreign country, just to be part of the public debate and to get public information. That is just plainly wrong.