Because the character he represents in the new timeline, in relation to the franchise, is "Dracula", the series antagonist. But LoS corrects the bit of a plot flaw following Lament of Innocence, in that Mathias names himself Son of the Dragon, as opposed to just Dragon. In another thread it was suggested he considers himself akin to the antichrist or the son of Satan, with Satan being the Dragon and him being the Son of the Dragon, but IMO it was just IGA not thinking things through.
In LoS Gabe claims "I am the Dragon". So the only logical name here is Dracul.