One good reason, demonstrated almost everywhere that hashtags exist, is that once you have them, it becomes trendy to play games with them involving dishonest tagging or jumping on a popular tag with no meaningful contribution (often both together).
Hashtags are also visual noise, though that's probably easy to manage, e.g., by allowing hashtags as part of the text entry, but then treating them as tags but not displayed parts of the comment text.
Hashtags are also visual noise, though that's probably easy to manage, e.g., by allowing hashtags as part of the text entry, but then treating them as tags but not displayed parts of the comment text.