Melty Blood Type Lumina director Kamone Serizawa, who appeared on stream, also explained why Type-Moon and French Bread decided to add Saber to the game, I translated his comments below.
The final reason is that despite being a fighting game, Melty Blood Type Lumina has few characters who are into “fair & square” fights, traditional knight characters with a weapon, so adding Saber would fill in that spot.
But since French Bread saw Tsukihime Remake was getting re-announced and released soon, they asked Type-Moon to let them make a new Melty Blood.
Before the reveal of the two to three last characters playable in Melty Blood Type Lumina, icons for the characters appeared on digital stores, which leaked that Saber from Fate/stay night would be in the roster – now, Type-Moon officially confirmed Saber with a new gameplay trailer on the live stream Type-Moon Times vol 6.