Music CDs are .wav files which are not compressed. In order to fit more onto a disc, audiobooks tend to be encoded in .mp3, .wma or .aac compressed formats, and it requires processing power to uncompress them on the fly.
CDs came out in 1982, at which time not even the fastest PCs had enough CPU power to play .mp3 files in real-time, so it's pretty unreasonable to expect an old CD player to just include fast enough hardware to be able to play them. And many later players could play one of the compressed formats but not the others (or variants like variable bitrate .mp3), so it also depends how the audiobook was encoded.