What are the properties that prevent large objects (like Quaoar, Varuna, and Sedna) from being easily observed

Question

AND...

Why is it considered likely that there may be other Pluto-sized objects in these two regions that have not yet been discovered?

Answer 1

They are not large - they are smaller than our moon.

But instead of being ~400000km away, they are more than 40AU (or 6 billion  km - that's 1500 times further away. So they look dim in reflected starlight, and even in the far-infrared, where they should emit light, they are dim.

Answer 2

Heres a site you can read but i believe its its size i might be wrong though so i recomend you read up on this