re: What are your sources for this statement?
In addition to the fact that the technology to find such distant and faint
objects wasn't really available until a decade or so ago, as Buck Galaxy
said, the other factor is that many astronomers - and not just those
upstart American ones, either - were not thinking of a huge belt of
objects in the vicinity of Pluto. Instead they were thinking of one or
maybe two large planets beyond Pluto, which they did not really know
how to classify and even looked on it as an abberation of sorts.
And as for comets, most of them were thought to be in the Oort Cloud,
which was really far away (maybe halfway to Alpha Centauri at its outer
edge), and thus beyond the capabilities of even current astronomical
technology.