Hello,
Lets look at the picture as a whole, and hopefully provide some insight into how DLC works in the normal of the industry.
First off, it is natural for people to move off projects as products get finished. Second most of the time DLC is not all made by the same team. Third the industry has shifted such that there is massive backlash for day 1 dlc so that cut things down.
ME3 had 4 pieces of story DLC. From Ashes was a day 1 DLC, where a very large team (comparatively) was able to churn out a DLC fairly quickly. For DAI I believe the majority of people took vacation during this time instead (because of backlash of day 1). Next up with Leviathan, which was produced by a reduced 'main game' dlc crew. Next up, Omega it was produced out of Montreal instead of the Edmonton studio. Lastly was Citadel which once again came back to the 'main game' dlc crew.
Looking at the release dates for Edmonton
ME3 Initial Launch: March 6, 2012
From Ashes: March 6, 2012
Leviathan August 28, 2012 (5 and a half months after launch)
Citadel: March 5, 2013 (1 year after launch, 6 and a half months after Leviathan)
DAI for Edmonton
DAI initial launch: November 18, 2014
Jaws of Hakkon: March 23, 2015 (2 platforms)
Jaws of Hakkon: May 26, 2015 (3 platforms)
So Montreal this time around have been working on ME:A they have not been available for DAI DLC thus why it looks slower compared to that of ME3.
(I was not around for games prior to ME3 so cant comment on them)
As for lack of advertising, it is hard to definitively know release dates with DLC since if there is a problem that crops up it might be harder to solve than when you are shipping a product and have literally hundreds of knowledge experts that might have an idea what is going on.