I think this is mostly because ESO is a very compartimented game, DLC wise - because you can play the game and some DLC without having other DLC.
So, some people not owning Thieves Guild / Dark Brotherhood DLC may think that they get a bit robbed if for, say Gold Road, ESO announce new heist / sacrament instances - they would litteraly pay for something they won't have access to. And for ESO themselves, they may think it would not be a very good move ( if even a legal one ? I don't know about that aspect of the question ).
Just like the dailies in Southern Elsweyr, wich ask you to go in other regions to fight the influence of the New Moon cult - all of these missions are in base game regions. Just like the story quests for Tales of Tribute, same thing. I guess the antiquity system is a bit borderline in that regard, but it might be the one exception.
The commercial model of ESO impose some limitations. If the TG / DB stories would be expanded in the future, it would most likely be an entirely new thing, in many ways separated from the original contents, because it would be kinda borderline to ask customers to buy old stuff in order to be able to play the new stuff, especialy if it's not an established thing - and I don't think it is in ESO.
Also, adding new instances for a rather limited and timed-out activity might not be the best use of ressources.
DenverRalphy wrote: »All that is easily avoided if a DLC containing new DB/TG content included unlocking the original DLC with the purchase of the new. Which I think ZOS would be willing to do given that they've given the TG DLC away for free in the past.
tsaescishoeshiner wrote: »[...] Also, new heists/sacraments could use new technology we have like grappling hooks, and other things from dungeons.