Because on paper he shouldn't be able to beat T'Challa. The current vibranium suit really should make it tough for Killmonger to hang with T'Challa in a straight up fight. And T'Challa should all around have better resources and gear.
The weird thing about Killmonger is on paper at least, he should be the underdog... but hes the underdog that has actually managed to be more sucessful against T'Challa than 99% of the other comic book villains are with their super hero nemesis. Most villians pretty much always lose. Killmonger often manages to actually win. And that's what sort of makes him unique. But as T'Challa overtime has had his share of upgrades, I think it was necessarily for Killmonger to do the same.
T'Challa had that suit when he faced Killmonger under Priest, and it wasn't a factor then.
Killmonger's a smart enough cookie to be prepared with an answer to the suit when it comes to battle. A lot of T'Challa's foes should be.
IMO, what he needs is better motivation. Blaming T'Challa for the actions of Klaw is the type of motivation we only see in comics. MCU Eric was compelling, because they tossed that.
Killmonger should want more in life than to kill T'Challa and take the throne. I wasn't crazy about it when I first read it a few years back but I think Hudlin had the right idea with giving N'Jadaka his own nation (even if it didn't last long). Erik is the way he is because he has severe abandonment issues and is looking to destroy an outside world that he feels is fundamentally corrupt. Instead of than turning all that rage towards one man it'd be great character development for him to admit all he really wants is a home to call own, Wakanda be damned.
As Coates hinted at in his run, Niganda was abandoned by their northern neighbor when the Wakandans took Alkama Fields and let them fall prey to outsiders. I feel he'd identify with that and want to fashion himself as their hero, which should be his new status quo. He wouldn't even need to run the day to day administration, he could be like Silver Sable and take merc jobs that help him fund projects while he lets his girlfriend Madame Slayy deal with the economics. And if you gave him his son (who I think should be biologically his) you can explore a new aspect of him as a villain with his own family.
Killmonger has massive potential as a villain for BP and all types of espionage-based heroes like Cap or Daredevil. The issue is that he lacks compelling motivation and characterization because writers default to the "I want nothing but the throne" **** which is boring.
His answer to the suit was to have his own suit... which is fine, except T'Challas suit should be better. He's smarter and has more resources. I can buy a guy like Doom being able to keep up (or even at times top) T'Challa in terms of gear and tech, but Killmonger in theory should be at good couple steps behind. He's smart and has some resources, but not to the same degree.
It's why I think he needs some sort of power upgrade. Matching T'CHalla with gear and tech in theory should be a losing proposition unless it's completely one sided.
As for motivation... I think Rise and the Killmonger mini somewhat took care of that. It's something they definately tried to address to keep up with the movie version.