Every beginning D&D player has taken a look at these homebrew classes and thought they were cool: I'm talking about the necromancer class, the witch class, the death knight class and the tavern brawler class. However, these aren't very popular at the table, and most Dungeon Masters don't play with them. Dungeons and Dragons is designed to contain the game in a handful of classes, and create specific types in a subclass. A necromancer can be played very well with a Necromacny wizard, and a death knight can be a paladin multiclass. Unless your content gives new value to the game and isn't "just another spellcaster", it's against the game's design and won't be added to the homebrew list.
When your homebrew gets declined with this reason and you wish to resubmit your content, please try to create your homebrew content in a different manner. Your class may fit as a subclass, and your race may be an extra subrace of an already existing race - or perhaps your ability should be reformed into a spell.