Marvel's Ant-Man is quite a divisive figure. Aside from his name, which doesn't inspire the same awe and/or respect as other names like Captain America or Thor, the very first Ant-Man - Hank Pym - is mired in controversy and fan derision. This stems from his backstory and a ghastly negative reputation that he cannot quite seem to shake, despite decades of attempts by both himself, his fellow superheroes and even his many writers. His lack of focus in the MCU is a direct result of this - and so is his death.
Hank Pym in comics is known primarily for two major events (aside from his origin story, which began as a one-shot in which he invented a shrinking device; Stan Lee thought the character had too much potential to only use once): his hand in the creation of Ultron and a single moment in the comics in which he hit his wife, Janet Van Dyne. In The Avengers #213, Hank Pym was under the influence of chemicals that forced his personality to fracture, exacerbating his already-worsening mental health issues. Nevertheless, the panel depicting him slapping Janet in the face (an image which editor Jim Shooter says resulted from a miscommunication to the artist, and was never meant to have Hank hit his wife) would go on to live in infamy.
In the Rage of Ultron series, Hank Pym merges with his creation and becomes Pymtron. It is never entirely clear how much control Pym has over Ultron's consciousness, or vice-versa. But nevertheless, the ending finds Hank Pym dead, and he's been absent from the main books ever since. As the 2015 film Ant-Man in the Marvel Cinematic Universe was a passing-of-the-torch film from Hank Pym to Scott Lang, Marvel saw the opportunity to gradually (0r violently) switch out Pym for Lang in the 616 continuity, and did so as soon as they could for the sake of synergy between comics and films.
However, thanks to the timing of the release, the death of Ant-Man confused fans to no end. At the same time, the Secret Wars event was in full swing; full of reality-warping, multiverse-changing narratives, Secret Wars led some Marvel fans to believe that Ant-Man's death was not canon and thus never happened. Some of his out-of-character actions, like arguing the fact that artificially-intelligent life is not "real" life, didn't help fan theories and expectations.
All this led to a gradual realization of Ant-Man's death; fans didn't understand that he was dead until after the series premiered. While Hank Pym was under the influence of a mind-altering substance when he lashed out in anger at his wife, the panel caused him to be hated by fans (even though in the comics later on, Pym was truly apologetic and made great changes in his life to ensure such a thing would never happen again, and the two did repair their relationship). While some comic books like The Ultimates turn Hank Pym into a cruel, vengeful character who actually enjoys torturing others, fans of Ant-Man remember the real Hank Pym is - perhaps - not as irredeemable as internet discourse suggests.