// why has it took so long to reach the majority of the national newspapers? //
Because the City of London Police only issued an appeal for help with this investigation yesterday. This is not really a news report into the crime, it is an appeal for information by the police.
https://www.cityoflondon.police.uk/news-and-appeals/Pages/Police-release-shocking-CCTV-footage-of-man-being-kicked-and-punched-by-gang-in-an-unprovoked-attack-on-Tower-Bridge.aspx
Even if the newspapers knew about this incident from a souce other than the police, when a criminal investigation is underway, there are reporting restrictions imposed. I assume that the fact the Police are trawling for clues from the public, that their investigation has got nowhere in the last two months.
Two of the men were riding Boris Bikes, which could have identified them, but presumably hasn't.