I know it is weird. The current BGM is much more amazing. But, the original music companied my playing for nearly two years. It is the kind of music that familiar and comfortable, and I’m already tempted to ask whether it is possible to hear it again. I love the game, especially the music. It was so breathtaking, just listening it.
So, if anyone has any advises, please let me know. Thanks. 
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.
I agree, I rather got used to the old music… though the new purpose-made music is excellent…
If you need a quick fix, you can hear it in the background of many of @Tacet’s older videos. 
Or start a console account. We still have the old music (for now).
Well I have a little bit of a lead, but it’s not much. I recently noticed that some of the old soundtrack for this game was featured in a YouTube video by Channel Awesome entitled “Batman v Superman - Nostalgia Critic” at around the 1:53 mark. It looks like Channel Awesome and the game developers might have gotten their music from the same source. Maybe searching through the channel awesome community might prove useful, but things still look bleak.
Channel Awesome forums: http://channelawesome.com/forums/threads/music-from-video-batman-vs-superman-please.57553/