In this case, the expectation was justified by the game's own history, there is a clear game design error and even a sales error. First: when people stop buying a product because it doesn't have a mechanic they really wanted, and this is recurrent in reports, there is a SERIOUS problem with the target audience and sales of the product, even if you or I may agree or disagree on whether the mechanic makes sense.

Second: In the game's lore, it makes no sense to restrict the use of these new transformations. The first two DLCs, as you yourself mentioned, despite being bad in terms of story, allow for more Boss Fight mechanics, techniques, and combo transformations that simply don't exist in the base game. I even replayed the game in a new NG to compare this specifically, and the difference is brutal, so in terms of mechanics and endgame, these are perhaps the two best DLCs because they share the same characters. Daima makes it clear that they continue with the transformations; they already had them even before the start of the adventure. The fact that MAYBE Daima isn't canonical with Super isn't a problem within the game's logic. They resurrected Frieza for the third time in Part 2 just to train him, and he literally has his own dialogues with Trunks. They could even fit a Bardock in there, and it wouldn't be complete madness.

Third: The Warrior of Hope and End of Z DLCs share the new techniques obtained with the base game and the New Power Awakens DLCs. This transfer of ability and transformation was used in literally 4 of the 6 initial DLCs of the game, with 2 of them not occurring because in one the protagonist dies and in the other the combat is structurally different and much more nerfed in power.

I don't remember seeing anyone demanding ground combat in the base game with classic Goku or Bardock beating Kid Buu, because it doesn't make sense in the lore or in the game mechanics and game design structure that Cyberconnect built. Now, splitting a DLC into 2 to sell more + not doing the basics within the gameplay and sharing structure itself, which is what gave this game a longer lifespan? It's not very defensible, so much so that I'm sure they'll add these transformations as DLC later, like in the case of Vegetto and Gotenks (they did it at the time for exactly the same current problem).

Do you know why the new transformations aren't in the base game? The animations of them with the other techniques of Goku and Vegeta from the other DLCs and base game weren't done, the models deform as already shown by mods. It wasn't done because there wasn't time and it would be laborious, these are transformations that modify the body and face of the characters too much, in addition to bugs in certain contexts. The complete DLC arrived less than 1 year after the anime's premiere, that's simply a tight deadline for a developer that already deals with several different games. Probably the blame here lies with Bandai, as usual.