Because it is all pretty much required to take place during the story rather than post story and the ending destroys any desire to revisit the main story.
You know all endings are shitty so who cares if you save Omega?
ME2 used the Shadowbroker as a bridge between the two games, that is far more interesting and it works perfectly fine with the ending of the game.
The endings might be shit, but nothing was wiped or completely eradicated in the end(well, if we ignore Geths i guess), so why making such a big deal about "Why should i care about Aria or Leviathans?". The Citadel still exist, Mass Relays still exist, Omega still exist, Tuchanka still exist, the ME races still exist, so what's the problem?
Might as well say "Why should i care about Overlord in ME2 if I know the Reapers are gonna fuck shit up in ME3 anyway?"
Honestly, i buy DLC for their gameplay and to see new areas and characters and storylines. Not because "Oh, but this and that happen later, so why bother?". If i was thinking like this, i would have never bother buying something like the Missing Link for Deux Ex HR and miss a good DLC.