A Meissner effect demo is usually seen as an easier and stronger proof of superconductivity than a resistance measurement, which is a delicate task and susceptible to experimental errors.
First, one cannot measure superconductivity with an ordinary ohmmeter. Electrodes, wires and the ohmmeter itself are resistive, the meter can never show zero ohms. So you can't just look at the screen read-out and say there's superconductivity, you need to set an experiment up to do it manually, with a current source and a voltmeter to measure the IV curve across the material [1]. Even then, the voltmeter will never show "zero" volt because of noise, such as thermocouple effect, triboelectric effect, or electromagnetic interference - which need to be minimized during the experiment and removed during post-processing. There are also the problems of sample preparation and purity as others have noted.
First, one cannot measure superconductivity with an ordinary ohmmeter. Electrodes, wires and the ohmmeter itself are resistive, the meter can never show zero ohms. So you can't just look at the screen read-out and say there's superconductivity, you need to set an experiment up to do it manually, with a current source and a voltmeter to measure the IV curve across the material [1]. Even then, the voltmeter will never show "zero" volt because of noise, such as thermocouple effect, triboelectric effect, or electromagnetic interference - which need to be minimized during the experiment and removed during post-processing. There are also the problems of sample preparation and purity as others have noted.
[1] It's basically the same 4-wire Kelvin sensing used by all milli-ohmmeters. But to characterize superconductivity, you need to do even better. https://en.wikipedia.org/wiki/Four-terminal_sensing