Comparing Black knight to Ephraim is daring. He's been in more than one FE Game, a widely recognized antagonist, and has a major fanbase. Gimme a reason for him not to be in Brawl.
For one, his fanbase is much smaller than you think. At best it can be referred to as a cult following. Larger than Ephraim's, sure, but thats because Ephraim hails from one the two least popular Fire Emblem games ever.
The major reason he'll never be playable is that he is a villain. The Fire Emblem series is a diverse, constantly changing one, with new characters almost every game. This puts new, more or less equally important protagonists into the games quite often - the Lords - and it is those characters who are most important to the series, most popular, and who therefore make the best representatives to be playable in Brawl. The villains, even the main ones, are secondary considerations at best, and as you'll see in a moment, the Black Knight isn't even a main villain.
Further, the Black Knight has little importance even to his own games, let alone to Fire Emblem as a whole. He is a secondary villain, subservient to the true villains of his games, and mostly simply gives Ike a personal sub-plot of tragedy and revenge. Outside of Ike and Mist, none of the characters care any more about him than they do about any of the other enemy generals. His antagonistic role with Ike may make him worth being a Subspace Emissary boss, but certainly nothing more.
Theres also the fact that hes from the same games as Ike. Because Fire Emblem is a diverse series, including multiple characters from the same games would be limiting its representation and failing to represent the series as a whole. One cannot simply including noting but Path of Radiance/Radiant Dawn characters, else its only those two games that are represented, not Fire Emblem. This is in contrast to most other Nintendo series, where the major characters are constants, so the whole series can be represented just by taking a few individuals using their most recent appearance and abilities (see: Mario, Zelda, etc).
Basically, the Black Knight lacks the most important things to recommend him, popularity and importance to his series, and has several things against him, being a villain in a series full of heroes and being from an already-represented part of a diverse series. You really need to lack knowledge of the series in order to take him seriously as a potential playable character in Brawl.
Zevox