She didn't say any of those things. And she's right.
Even if you take the political slant out of how/why news media covers things, and the fact that we are constantly consuming content relative to the days when our only news came via the newspaper and the 6 o'clock bulletin, there is a very specific reason that you 'hear more' about certain topics currently.
News media makes its money online. Ad revenue is much lower than it used to be, so the only way they can keep the lights on is by creating content that people will click on, that will appear high in Google's search results, that will be shared on social media and that will be boosted by social media platforms' algorithms.
Online articles are written/edited by SEO (search engine optimisation) specialists. The same outlet will produce multiple pieces of content on the same story.
None of this is to say that knife crime isn't happening, or isn't an issue...but the reason you're 'hearing more' about it is because it's a hot button topic that's perfect for monetization.