the mastering is done at higher sample rates which lets more bandwidth be recorded. due to CD being lower sample rate there is not enough room for the higher bandwidth. a full mastering of a single track could be 300mb and there is only 700mb on a CD so something has to go. LP's are analogue so no sample rate but are still bandwidth limited just not as bad as CD.
it's only recently i have discovered that CD's can sound just as good apart from Led Zeppelin which sounds blooody horrid on CD for some strange reason.