Here's why:
* Monster Reborn is a Spell Card that allows you to Special Summon a monster from your graveyard to the field.
* Synchro Monsters are summoned using the Synchro Summon mechanic. This mechanic requires you to use Tuner Monsters and non-Tuner Monsters as materials.
* Monster Reborn does not follow the Synchro Summoning rules. It simply Special Summons a monster from your graveyard.
Therefore, you cannot use Monster Reborn to bring back Synchro Monsters from your graveyard.