Because it allows the storyteller to expose another side of the hero in a way that is easily accessible by the audience. Every blockbuster action movie has this for the same reason, as well as most dramas. There are ways around it, such as having a story with no women, or making the movie entirely action (thus with shallow characters), or having a sort of non-romantic love between two same sex characters. Generally, to make these good it takes more artistry than 90% of writers have. Generally, writers that are that good are working on their own artsy projects, and not on summer blockbusters and action movies.
You weren't just curious. You specifically and repeatedly suggested he works better as a co-star, simply because you've never read him. You can see how that would really suck for someone who is a fan of a character to hear, right?
Let's help you out then:
Reasons Black Panther is awesome:
1) He's Black.
Let's admit it, it's there, and it works. Not only does that make him stand out as unique, but it also brings in all the positive stereotypes associated with black people. He is expected to be a physically capable badass, for instance. It also opens the door for some great actors who haven't gotten a shot at being a superhero before. Chiwetel Ejiofor. Djimon Honsou. Michael Jai White. Adewale Abbaje-Akinouye. Anthony Mackie. To say nothing of young up and comers like Aldis Hodge. Toyanga Chrisi. Michael B. Jordan and so on. With the supporting cast it's even worse when you talk about accomplished actors who have no danger of double dipping because they've never gotten to be in a superhero film before. Oscar winners and mess. Movie poster in the bag.
2) He's a king.
This means he has his own country to run, which invites all the political storylines you might be familiar with from James Bond to Enemy of the State to Executive Decision to Air Force One. The President from Independence Day? That's what he does on a day to day basis. Abraham Lincoln Vampire Hunter? Been there done that, since 1976. This also makes his supporting cast incredible, as it includes things like The President of the United States, his "harem" of ceremonial wives in training, the Dora Milaje, a situation/war room full of people to argue foreign policy while they have their fighter jets scrambling. International incidents. Royal court coups. Pomp and all manner of circumstance. Epic storyline in the bag.
3) He's one of the best hand to hand fighters in the world
, on par with Cap, Wolvie, Elektra and the best of the best. Plus, he's got some plant-based superhuman stats. Imagine if Avengers Captain America had MCU Black Widow's fighting agility and skill. That's what Black Panther is on. He's not just peak human (and past peak human in agility) but he knows how to use it better than any other superhuman. With an artistic fighting style based partly on capoeria, his fighting is also unorthodox, which makes him hard to beat on paper, and interesting and exciting to look at on screen. Fight scenes are in the bag.
4) He's got gadgets out the wazoo.
Cap's SHIELD is the kind of low-tech thing Wakandans give away to people they like. Their swords cut like lightsabers, their computers are ten years ahead of everyone else's and T'Challa's costume absorbs energy and sound to make him the stealthiest amongst the stealthy, in addition to anything else he invents. Yeah, he does that too, Reed Richards level at times. Geek out moments are in the bag.
5) He's in Africa.
Which provides a lot of rich locales ala Blood Diamonds, District 9 and Last King of Scotland. Not to mention when he travels to other nations to handle business. Also, it's cheaper to shoot in Africa, if you can get the crew to go. Cinematography is in the bag.
6) He's got a fair rogue's gallery.
Better than Cap or Wonder Woman's, not as good as Spider-Man or Batman's. Ulysses Klaw, his sound-manipulating origin villain tops them off, but a close second is Erik Killmonger, the homegrown mastermind that can actually take T'challa in a fistfight. M'Baka (Man-Ape) is a bit like his venom, the stronger dumber version of himself, basically, also homegrown. Add to this other sundries like the elemental trader Moses Magnum, traitorous brother Hunter, fickle Panther God Bast and all the assorted malcontents that aren't smart enough to run things, but get employed by Klaw and Killmonger from Malice (former fiance of T'Challa) to Rhino (borrowed from Spidey), to Radioactive Man and Baron Macabre. Trilogy's worth of epic climactic battles in the bag.
7) Can they make it exciting, though?
They already know how to make it exciting. They just have to decide to fit it in the schedule and hope people will actually watch instead of guessing out of the blue that he works better as a sidekick.