What is responsible for the poor fidelity of your mp3s?

When I recorded my vinyl LPs 20+ years ago - several by Nic Jones, but not that one - I made both mp3s (128kbps because disk was expensive then) and CDs (CD-DA). The CDs are probably as good, quality-wise, as the LPs: I had an expensive deck in those days. They do have some of the the surface noise from the LPs.

Whether the mp3s sound as good as the CDs from which they were originally encoded depends on what I play them on. If I use my main computer, which has a powerful CPU and good decoding software (but crappy speakers) the mp3s sound as good. On the puny media player attached to my hifi, or on a tablet played through a bluetooth speaker, they don't.