There are people that worked on the new song, so money and effort was spent on it. There are reasons for why tripwire want their game to be this way, its their vision after all. Sometimes one also shouldn't cling onto the old things, and needs to create new exciting ones, in hope that they will be even better than the old ones.
But the old shouldnt be forgotten, as theres a lot of emotion, and SPIRIT in the old melodies, The overall idea of this game aged and evolved, and the players do see these things, things that theyve heard from way back when they first played kf1, as a symbol that this is something genuine and with continuity.
With all of that sentiment being said, considering the need to code, and script, and complicate some things, i dont feel that theres any thematical reason not to have more than 1 main menu song, as they could share the play time based on some conditions. Things such as the new one playing as the game boots up for the first time of the day to excite the player, and the old one on consecutive launches? Or the new playing on game boot up, and the old when player disconnects from a server to return to main menu. Or the above things combined with some additional probability, idk.