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Because the levitation is a much better proof. It can only levitate if the resistance is really exactly 0.

While measuring it somehow always adds errors, because the probes themselves have resistance. And then you can't be sure if it's 0 or just really low.

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There are measurement configurations that can be used to eliminate contact resistance (such as a van der Pauw measurement). Resistance, and electrical measurement in general, is the true proof - the Meissner effect is secondary and more confusing as, since this paper shows, a small ferromagnetic phase can give rise to the same effects.

You can't completely eliminate contact resistance, and all instruments have a noise floor, so electrical measurements don't constitute "true proof", they are an element of proof among others. The Meissner effect is perfect diamagnetism which can cause levitation and is not like ferromagnetism which only can allow a piece of material to stand up to align itself with field lines (and which is what we've seen so far).

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