When I say tradition, I mean that Marvel and DC have characters that have decades upon decades of history piled upon them and are practically at the level of mythic characters of old. New characters can rise to the top, but it takes time and energy. That is one reason why Spawn, a unique black character, was able to fly to top of the charts; he was in his own universe and was not pulled down by being in a sea of characters who had been around since the sixties, if not long before. Black Panther could become popular with the right stories and promotion, but I don't think that he will ever reach the level of Spider-Man which, in the money-driven world of comics, is important.
And as far as popularity goes, money talks. If Marvel has to pick between cancelling a Northstar series or a Wolverine series, which do you think will probably get the axe? It's not right but that's how the business works. Niche characters can succeed but they are always in danger of being shoved aside in favor of more established characters.
Now don't get me wrong; I want to see Marvel heavily promote minority characters as long as the stories are good. I do think, for example, that Black Panther does have the potential to become a heavy hitter in the MCU. That being said, we do have to be realistic in terms of business, and that newer and less well-established characters do have a massive uphill battle when it comes to having long term viability. Massive initial success is important, but it needs to be followed up with solid storytelling. Captain America could have several years of ass quality stories and he would still be given the same treatment as he always has, but if someone like Kamala began to lose quality and followers, her future would be in serious jeopardy. At the beginning of the 90s, Venom was everywhere and was almost A-list, but several years of aimless storytelling crippled the character's development and led to him being something of a lightweight until 2011. New characters, minority or not, need to seriously prove themselves, especially in the arena of the Big Two. Like I said, I want bold, new characters and stories, but if that is to become a trend, it needs to be on a base that emphasizes not only good storytelling but the reality of the business world.