Determining the shape of ʻOumuamua via stellar occultation
Observing star occultations is dependent on the object passing in front of a star.

During its short time in the inner solar system, ʻOumuamua didn't pass in front of a star (from the point of view of Earth) It is now beyond the orbit of Saturn. It became too dim for Hubble in about January 2018. There are no forecast occultations of any star (and its orbit is probably not well enough determined to predict any, given the observation of non-gravitational and not well-understood accelleration). Any occultation that did occur would only happen for a very short time and only be visible in a very narrow strip of the Earth. We can't set up large telescopes every 100m on Earth, and constantly observe all the (very dim) stars that might possibly dim further for half a second.

By contrast, Ultima Thule is a much larger object, with a well-known orbit, and the possibility of forecasting the exactly when and where an occultation of a reasonably bright star will occur, and set up observation stations on the narrow strip that is predicted.

So while an occultation could have told us about its shape, none occurred and none are expected.