Elder Scrolls Anthology: Contains the five main series games on CD + the construction kit for Morrowind (but not Oblivion!), and the major DLCs for the latter three games. Also, after some pressure, they added Steam codes for the GOTY versions of Morrowind and Oblivion, so you can get the 'minor' DLCs for these games as well. They do NOT include the spin-off games Redguard and Battlespire.

Fallout Anthology: Contains the four main games (Fallout 1-3 + New Vegas) and ALL the DLCs for New Vegas and Fo3. Also inculdes Fallout: Tactics spin-off game.

So - the spin-off games from the TES series were NOT deemed worthy of inclusion in the Anthology, despite them being referenced in newer games so their canonicity should be without doubt. But the Fallout: Tactics spin-off game from the Fallout series IS included, despite Bethesda having said on more than one occasion that they at best consider it only semi-canon...

Am I the only one having trouble seeing the logic here?

I think it's because ZeniMax Media Inc. and Bethesda Softworks started selling An Elder Scrolls Legend: Battlespire and The Elder Scrolls Adventure: Redguard in August of 2015 on gog.com.

One of the gog.com employees said they will continue to keep on supporting and updating The Elder Scrolls: Arena, The Elder Scrolls II: Daggerfall, An Elder Scrolls Legend: Battlespire, and The Elder Scrolls Adventure: Redguard.

So the support of updates I guess is one of the reason why I think An Elder Scrolls Legend: Battlespire and The Elder Scrolls Adventure: Redguard are not included in the The Elder Scrolls Anthology collection, That and gog.com decided to get them in working condition before ZeniMax Media Inc. and Bethesda Softworks sold The Elder Scrolls Anthology collection.

Oblivion's construction kit can be downloaded from the internet for free. It has always been that way.
Morrowind's http://www.uesp.net/wiki/Morrowind:Official_Add-Ons have also always been free on the internet.

The anthology never claimed to contain every TES game, it's just the main series.

^ This too.

The old Fallouts have been on GOG since longer, AFAIK

I know it's why I said this next part in my comment.

"That and gog.com decided to get them in working condition after ZeniMax Media Inc. and Bethesda Softworks sold The Elder Scrolls Anthology collection."

I think that's one of the reasons.

Oops I meant to say before ZeniMax Media Inc. and Bethesda Softworks sold The Elder Scrolls Anthology collection,

Some People prefer having the Physical Game Discs instead of a Digital Copy. If I did not already have all of the Games between Steam and GOG, I would buy the Anthology just for the Case. I bought the Elderscrolls Anthology as a set of backup play discs for Oblivion and Morrowind.

I think it's about Quantity.

Elder Scrolls Anthology: Arena, Daggerfall, Morrowind, Oblivion Skyrim

Fallout Anthology: Fallout, Fallout 2, Fallout Tactics, Fallout 3, Fallout: New Vegas

That's five games a piece. If it were just the main games, Fallout's anthology would only come with three games, all of which you could probably buy for less than half the price of what the anthology would sell for.

New Vegas is considered a part of the main series, so four games.

That aside, at the time of TES Anthology release, Redguard and Battlespire weren't available for sale. They were re-released almost two years later. Bethesda couldn't just add them to the anthology even if they wanted to because they weren't playable on modern PCs.

Indeed, the CK for Oblivion never been released on disc, it always been a http://www.elderscrolls.com/oblivion/downloads/.

What pressure? The Steam codes were there from day 1.

True. Most likely a technical issue, cause they are old DOS games, and to be honest, getting Redguard to work well on a modern computer is a challenge they couldn't be bothered with it seems. While the GOG staff took on that challenge.

While Fallout Tactics is a 32-bit Windows executable that use DirectX, just like Skyrim, despite being released a decade apart. There's no technical issue to speak of.