This type of object could only really have been seen when it was. It was picked up as part of a full-sky survey that year, the firsts time such a thing had been done at sufficient resolution to detect an object like 'Oumuamua.

A number that I saw quoted earlier was that it is expected that around 3 of these things go through the solar system every day. This one just happened to be the one caught by the survey, which itself required a bunch of coincidences since it means catching multiple pictures of the same object in different locations so as to establish a trajectory, so an ability to detect is not just resolution, but also timing and orbital trajectory.