One possible explanation is, randomness is not going to hurt you, so might as well use it. Also, if we are hoping to prove impossibility, it only makes the result stronger.
One other explanation is if the obfuscators are defined to be deterministic, then to satisfy the strong virtual black-box definition (and the weaker indistinguishability obfuscation definition), it should be the case that all functionally equivalent circuits (that are of similar sizes) must be obfuscated to the same circuit, and this is difficult.
But, what if we are considering a class of circuits where no two circuits in the class are functionally the same? For example, a class of point circuits (where each circuit in the class evaluates to 1 on a specific point that is hard-wired in that circuit, and evaluates to 0 on all other points). Does it make sense to think of deterministic obfuscation in this case?