Because the levitation thing works with tiny fragments too whereas the 'zero Ohms' measurement only works when you have enough of it to form a long enough wire of it that you could tell the difference between 0 and 0.000001. Measuring very small resistances is usually done by letting a known current run through them and then to measure the voltage across the sample using known resistance leads. For a superconductor that has a tiny available sample which is already conductive even if it is not superconducting is fraught with error and may well give you the wrong answer due to a tiny measuring error.
But the Meissner effect is a unique signature of a diamagnetic material and will provide you some evidence even if the sample is tiny. So I understand why they have not yet resorted to other measurements, if there is no Meissner effect you don't need to continue with the hard work of trying to make a wire (which may well be a serious challenge for this stuff, the yield will have to come up significantly before that's a real possibility).
But the Meissner effect is a unique signature of a diamagnetic material and will provide you some evidence even if the sample is tiny. So I understand why they have not yet resorted to other measurements, if there is no Meissner effect you don't need to continue with the hard work of trying to make a wire (which may well be a serious challenge for this stuff, the yield will have to come up significantly before that's a real possibility).
hth