Well, the obvious answer, Chuck, is that Hammers have or, rather, had the knockdown lockdown. Properly arranged and timed a Hammer Warrior or a group of Hammer Wars could keep an opponent on their back and doing nothing for, well, forever. Knockdown was the best condition to inflict, hammers were the princes of all weapons, and, of course, that couldn't be allowed to stand so knockdown lockdown got hit hard. And the rest of their skills seemed to suffer to reduce the overall effectiveness of hammers overall.
Now, by chaining an elite knockdown into a "lose all adrenaline" knockdown you'll get 5, 6 seconds of knockdown. Which is a lot and nothing to scoff at but it's about all a hammer has going for it these days.
Hammers, as a weapon, are probably fine. They do more damage but they swing a bit slower. They do a bit more damage, on average, than any other weapon but to do so you lose the ability to carry a shield or a focus and all the mods that could be there, too. It's arguable but they're pretty much in line with the other weapons in terms of power.
Hammer Mastery, on the other hand, as a skill line, is incredibly thread-bare and poor and it's been that way for a long time. It just doesn't stack up with the other skill lines in terms of diversity, of utility, of raw power, or anything else even before you take into account that what little it does have has extremely high adrenal or recharge times to go along with that slower Hammer swing rate.
The why of it, I don't know. It's probably a case of the squeeky wheel getting the grease. There have been bigger problems elsewhere, more abusive things, more massive fixes needed with other skills and lines. Hammer have been "meh" for a while. Not quite awful, not quite great, just there. So, they get neglected as changes get made.