Warning: contains spoilers for Taskmaster #4!
Nearing the end of his mission to clear his name, the mercenary Avengers. Although Tony Masters served alongside other famous Avengers like Thor and Scarlet Witch, his participation in attacks upon Atlantis coincided with the same group's efforts to capture T'Challa and take over Wakanda. Although he realized he was backing a sinking ship and eventually switched sides, Taskmaster finds that years later, he's still paying for a mistake that forever blacklisted him from the most technologically advanced nation in the world.
Secret Empire saw a brainwashed Captain America acting as a sleeper agent for Hydra, manipulating government and superhero alike in an effort to take over America and establish Hydra as a world power. As cities fell under Hydra's control and heroes were captured or executed, Captain America attempted to maintain control with a new team of Avengers which included Taskmaster. When foreign nations like Atlantis or Wakanda refuse to submit to Steve Rogers' vision, his loyal agents attacked both countries, hoping to either force them into submission or overthrow their leadership entirely. This Captain America was later revealed to be a creation of Kobik, a sentient child version of a Cosmic Cube, and the real Steve Rogers and his allies defeated Hydra Cap and ended his takeover of the United States entirely.
In Taskmaster #4 by Jed MacKay and Alessandro Vitti, Taskmaster has been framed for Maria Hill's murder and, in order to prove his innocence, he has to collect the kinesic signatures of three high-ranking leaders of separate superhero organizations which include Agents of Wakanda leader, General Okoye. Parachuting down to Wakanda, Taskmaster is beaten, captured and brought before Okoye. He attempts to use his photo reflexive abilities to acquire her signature, pleading asylum from his enemies to buy time and revealing, truthfully, that the Black Widow is on his tail, intent on killing him for a crime he didn't commit. He is denied his request to work for Wakanda, as Okoye says, "Wakanda has no need of American mercenaries. We have not forgotten your role in Hydra's Avengers."
After being imprisoned for a short period while Okoye considers his punishment - and no doubt consults with Black Panther about the high-profile prisoner - Wakanda decrees that Taskmaster be handed over to American authorities, with Okoye making it clear that the nation is perfectly happy to send him off to what's likely his death as Black Widow closes in. Wakanda's grudge against Taskmaster appears to be a case of guilt by association. Although Taskmaster points out he only participated in the attack on Atlantis - while it was Baron Zemo and Dr. Faustus who attacked T'Challa - he was still a part of that same organization, and thus is clearly held responsible for the insult to Wakanda and its embodiment and leader, the Black Panther.
While it's General Okoye's decision - made after Black Panther and his country have neither forgiven nor forgotten his Avengers service.
