There was a discussion about this in this thread. In short, however, they had licensed the original music, did not own the rights to it, and were therefore restricted in how they could use it. For example:
It’s not clear if they no longer have the right to use it in the game, or if they simply think it would sound discordant to hear a mix of the old and new music. The only track I miss is the one that played when purchasing a new kingdom, but I understand that other people would have developed a fondness for other tracks.