Because they were getting really stale and they wanted to hold onto the franchise name anyways. People are nostalgic now, but when AC games were coming out back to back people were getting really bored of it.
When Assassin's Creed 3 came out, everyone was saying how this is the third time they remade what is basically the same game (AC2, brotherhood, revelations, then 3). People weren't impressed. Unity and Rogue didn't sell that well. Syndicate apparently also didn't sell very well.
So they basically remade the franchise into an RPG starting from Origins. They aren't that special, but honestly, neither was everything after Revelations.