They need a hero.

One of the big things, right after the huge announcement that TESO was a real thing and it was actually happening, was that lots of Elder Scrolls fans remembered that they had just licensed the HeroEngine, famous for The Old Republic. The implication was kind of scary - instead of Bethesda's usual method (IE building it from scratch) they would be borrowing another engine. In a Game Informer interview, game director Matt Firor explains the role of the HeroEngine with TESO, and here's the real answer as to why it was licensed;

"It takes a long time to write game engines, especially MMO engines, which are inherently more complicated than typical single-player ones. So, we decided to license the HeroEngine to give us a headstart. It was a useful tool for us to use to prototype areas and game design concepts, and it provided us the ability to get art into the game that was visible, so we could work on the game’s art style. [...] Think of HeroEngine as a whiteboard for us – a great tool to get some ideas in the game and start looking at them while the production engine was in development. "

They explain further about some of the ambitions of the Elder Scrolls Online and how they are approaching the "just another MMO" mindset fans have as well, and put some worried minds at ease.