Because there's the possibility of them being out of date? It's automated, so breaking news wouldn't be covered. For example, a few years ago, they could've ran a story about Michael Jackson being about to go on tour... which ended up cancelled because he died. His death would've rendered the previous story obsolete.
Are you planning to use the current music tracks from ITV Nightscreen with your new mocks? ITV2 Nightscreen have added a lot of new music tracks to it from the late summer. It could fit nicely with your designs with making them a lot more refreshing in the process. TIA.