In the faster versions the players would have a hard time keeping up. If you're not sure what this means, listen to the keyboard part again. At too high a tempo, it would be almost impossible to play.
In the slower versions, the players would have a different problem. Without a quick enough tempo it becomes easier to lose your place and the song can fall apart. Also, some instruments like the drums don't hold their sound for long. Listen to the slow version again and notice that the organ notes hold the entire time, but the drums fade away quickly.
Note also the slower version makes it easier to hear all the notes. This is why it is easier to figure out a song from a recording if you're able to slow it down. But all the versions you heard here were created with a computer (no actual instruments!). If you were to slow down or speed up the original version too much, it would sound really slow and dragging or like hyperactive chipmunks.
Listen to the next part of the song and you will hear it for yourself...
