My wife wants to know why GOG removed the game series Fallout (I remember it was available for sale once) and Why is not available series TES, her favorite games.
When Fallout 1, 2 and Tactics were sold here, the IP still belonged to Interplay. Then, Interplay and Bethesda got into some legal wrangling (over a Fallout MMO, I believe), which resulted in Interplay losing (or giving) the rights to everything Fallout related to Bethesda. Since GOG never managed to make a contract or agreement with Bethesda/Zenimax, the Elder Scrolls games were never sold on the site and the Fallout titles had to be removed.
That's also the reason no games from id Software (Doom, Quake) are here, because they are also owned by that company.
As a sort-of farewell to the series, GOG (and Interplay, I suppose) gave all three games away in December 2013, just a few days before they had to remove it from the store.