I think a big part of it for me was the perks. Almost none of the perks significantly change gameplay mechanics much -- they're small percentage tweaks to stats.

The firearms were pretty similar, other then the science weapons.

The game played in a pretty linear fashion, even though it was technically open-world. Not much backtracking.

I almost never stumbled across interesting things going on in the world a la Fallout. Just in cities.

None of those features individually made the Fallout series, but collectively not having them adds up.

Was pretty bug-free, which was nice.

It wasn't awful and it did share a lot of similarities with Fallout, but it didn't have the mixture that made Fallout "click" for me.