Well that's actually easily answerable. Doom's ascension to the throne could be fairly recent which may be why nobody has mentioned him. Latveria wasn't always ruled by Doom and as such one could create a film showing him taking over and establishing himself. His skills in the mystic arts could be from his mother or he's met the Ancient One and we've established in Doctor Strange that there is a ton of sorcerers out there who have gone through her doors. Doom's own armor can be understood as easily as it was to accept Tony Stark or Hank Pym.
Just stating he's new makes enough sense since even the source material explained it that way. Nobody really gave a damn about Latveria until Doom showed up be it when he was brought to America as a student or when he returned and took over.