Because you're not measuring a pure superconductor sample. To be able to observe the cliff on the IV curve you need enough of the current path in between the e

Because you're not measuring a pure superconductor sample. To be able to observe the cliff on the IV curve you need enough of the current path in between the electrodes to be superconducting, but certainly not all of it will be. This is true even in commercial superconductors, just with the ohmic losses pushed down even further.